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Adjusting Guitar Truss Rods 
This article answers common questions:
What is a truss rod?
How do I know if I need to adjust my truss rod?
How do I adjust the rod properly?
What do I do if I can't adjust it any further? When string tension is placed on an instruments neck, the neck will naturally want to bow forward with the tension, truss rods help us control and adjust neck bow. To put it simply, when strings are plucked they vibrate in somewhat of an elliptical pattern and that is truly oversimplifying it. Because this pattern is wider near the center of the neck and tighter where the strings are anchored, relief (bow) gives the strings room to vibrate without contacting the frets and causing a buzz. Because necks, playing styles, pick attack, truss rods and strings gauges vary, the "ideal" neck relief can also vary from one instrument to another.
I have repaired many instruments that played beautifully with a relatively small amount of relief but there will always be circumstances (the player, the string gauge, the guitar) that make a very flat neck inappropriate.
As with all set ups I must determine what the player desires and what the instrument is capable of and bring the two together to arrive at the ideal adjustment. 
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Tightening a truss rod forces the neck back and should reduce forward bow.
Loosening a truss rod decreases resistance and compression to the back side of the neck allowing the strings tension to pull more relief (forward bow) into the neck. Dual action truss rods These rods can add relief as well as straighten a neck. When turned counter clock wise the rod bends forward creating forward bow (relief) to the neck. These rods have small hex nuts that are welded to the end of the rod and unlike a tradition rod, the nut can not be removed. Dual / Double truss rods Don't confuse these with dual action (meaning a rod that can be bent both directions). Dual or double truss rods are most commonly found on 12 string or bass guitars for additional strength and rigidity. They are two individual truss rods laid side by side in the neck. 
Non-adjustable truss rods/ neck reinforcement - To strengthen the neck and control bowing, necks are usually inlaid with "something", be it a steel rod, graphite or other material for additional stiffness but not all necks have adjustable rods.
Two examples of non-adjustable truss rods on earlier Martin guitars. Checking Relief
 Check your necks relief- Here is a simply way to check the relief in the neck if one doesn't have a straightedge handy... the instrument strung to pitch fret a string on the 1st fret and where the neck meets the body. 
Checking Relief
By fretting a string on 2 points we can then use the fretted string as a type of straightedge and get a visual as to how much relief is in the neck.
Measuring the gap between top of fret and bottom of string gives us an indication of how much relief is in the neck.
Now observe the space between the fretted string and the point of greatest relief...usually the 6th or 7th fret depending on the length of the neck. This gap can be measured with a feeler gauge if need be (you can place a capo on the first fret to free up one hand). If there is no gap, this is an indication that the neck is either dead flat or in a backward bow.
If the gap is substantial the truss rod may need to be tightened to reduce excess relief. Determining the ideal relief The ideal relief for your instruments neck will depend on string gauge, playing style and the instrument itself. (I've probably said that like 3 times now ah?) So you're starting to get the idea that one size does not fit all and you can't take some arbitrary measurements and make them work for all instruments. But you gotta start some where...are we in the ballpark or way over the fence.
Light pickers, jazz musicians and the like may find .004/.006 gives the neck a very fast feel. The necks stiffness and willingness to flex can sometimes interfere with our desires though.
Those who have a moderate to heavy strum, like flappy extra light gauge strings, have a rounder f/b radius etc. may come to realize more relief (say .008-.012) may be necessary so that the strings can avoid buzzing. Once you understand how and when to adjust a truss rod you will be able to make incremental adjustments to arrive at your ideal relief and keep the necks relief finely tweaked. Some necks will require more frequent adjustments than others.
Changes in string gauges (tension) and humidity create the need to readjust the instruments truss rod throughout the instruments lifetime. Adjusting The Truss Rod
Adjusting your truss rod Word of caution: While I do not feel adjusting a truss rod is rocket science you should be aware that a broken truss rod is very bad news and normally...a very expensive repair. On inexpensive instruments broken truss rods are usually the kiss of death unless the rod can be removed without removal of the fingerboard. I am not trying to scare anyone, understanding how a rod works can save you from a catastrophe.  | If possible...first practice on a yard sale special. Tighten and loosen the nut and watch how it effects the necks relief. While I can certainly offer players some knowledge and understanding what I can't give you is experience. | | You may wish to make a mark on the truss rod nut, when possible, so you can gauge your progress or return it to it's previous position if necessary. | | Use the correct size Allen wrench or socket so you don't damage the nut. Martin's and many other acoustic guitars with truss rod nuts beneath the f/b extension use a 5mm allen head wrench. Gibson's and others with a larger acorn style nut at the peghead use a 5/16" nut socket. Taylor uses a smaller 1/4" nut socket. Many Fender Electrics use a 1/8" allen head wrench. | | Always start by loosening the nut first. If it is already as tight as it will go and you try to tighten it some more...pow! Adious truss rod, hello heartache. | | Make adjustments in small increments. A quarter of a turn would be allot of adjustment for most instruments. If your neck has a tremendous amount of relief in it and there is very little change after a good deal of tightening you're probably better off getting some advice. | | Adjustments to the truss rod are normally made with the instrument tuned to pitch. Without full tension on an instrument you can not judge your progress or effect. An exception would be Gotoh's side adjuster. There are also instances where I will clamp the neck into a backbow before attempting to tightening the nut. | | If you encounter resistance, think twice about what you are about to do. It is so inexpensive to have a truss rod adjusted that it just doesn't pay to take a big risk. | | Relief should be evaluated after each adjustment. Some necks take time to settle. | | Dry threads should be lubricated to prevent seizure. |
Typical Truss Rod Wrenches
And another word of caution: I can not tell you how many people have been given the impression that the truss rod is some magic action lowering "key", just turn it and the action gets lower. Unaware of its true function they starting turning the rod until the strings get closer to the fingerboard, many times placing the neck into a backbow. If an instruments truss rod is properly adjusted and the action is high, adjustments to the nut and saddle are in order. Continuing to tighten a truss rod after the neck is dead flat serves no purpose as the instrument will simply buzz as the strings vibrate against the hump in the center of the board that has been created by backbowing the neck. An Over-tightened truss rod - When a good deal of force is necessary to tighten a truss rod or if a neck fails to respond when the truss rod is tightened, its time to stop and ask for help. Greatly overtightening the rod can cause damage to the neck. I have seen some necks that were cracked at the truss rods anchor points (normally in the 1st to 3rd fret area). You should also bear in mind that excessive force may also merely be driving the truss rod nut into the wood (compressing it). When sensible measures have been taken without response it makes no sense to keep cranking away. Regardless of ones personal inability to stop and ask for directions...I would highly advise it at this juncture! :) 
Truss Rod Adjustment Limitations and Problems
On most instruments adjusting the truss rod is a simple matter once you have an understanding of how it works, but inevitably there are going to be exceptions.
One such exception is when we have already arrived at a maximum adjustment and still have not removed (or added) enough relief. You may encounter a truss rod that is completely loose and the instruments still does not have enough relief or one that is as tight as possible and the neck still has way too much relief.
In both situations it is best to have it inspected to determine the best course of action. When the neck is dead flat or in a backward bow I normally advise my clients to use a heavier gauge string in hopes that the additional tension may create the necessary relief. When heavier gauges will not do the trick it may be a candidate for a plane and refret. I can only determine the best course of action after inspecting it and undoubtedly, it is far easier for a luthier to straighten a neck with too much relief than it is to create relief in a neck that is bowing back.
Necks with far too much relief can sometimes be fixed by planeing and compression fretting. The use of fret wire with a larger tang causes a wedging effect and is sometimes enough to straighten or stiffen a neck. Repairing a neck that is set in a backbowed position is a whole 'nuther thing! I see this most often on 50's-60's Fender maple necks though they don't have the corner on the market. The instrument will have to be inspected to determine whether planeing is an option and one should rarely expect necks in this condition to act perfectly normal. Repairing Broken Truss Rods A stripped or broken truss rod is the kiss of death for most inexpensive instruments and the repair of such is usually only considered on instruments of value. In most cases, replacing a truss rod will require removal of the fingerboard.
The procedure can be quite time consuming on some instruments. The finish which is sprayed over the neck and fingerboard is chipped somewhat when separating the board from the neck. This can be a much bigger problem on necks with thick finishes or those with finishes that are more difficult to repair (i.e. U.V. cured finishes).
Because heat is required to soften the glue joint between the neck and board, celluloid inlays and binding can be damaged in the process, which will then require replacement. The heat may also cause the frets to spring free from their grasp.
Once the board has been removed I clamp it flat to reduce warpage or twist as it cools.
Depending on the instrument, some truss rods may be inlayed flush with the top of the neck, easily visible upon the boards removal or buried deeper and covered by a strip of wood. 
A Gibson Les Paul I have removed the fingerboard from... the truss rod still lies beneath a filler strip.
Whatever the case, it's now time to dig out the rod. Once it has been removed, the channel must be cleaned and the new rod installed.
Related Articles:
Martin Truss Rods (A comparison)
Neck Problems | 
