• Tightening a truss rod: forces the neck back and normally reduces relief.
• Loosening a truss rod: allows the string tension to pull more relief into the neck thereby creating a slight upward bow.There are also dual action truss rods that can creating opposing tension which adds relief.
• Determining the ideal relief : The ideal relief for your instruments neck will depend on string gauge, playing style and the instrument itself. Unfortunately I can not simply give you a single measurement or number which is appropriate for all applications, though .010 is a good starting point. 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.
  
• Changes in string gauges (tension) and humidity create the need to readjust the instruments truss rod throughout the instruments lifetime.
  Remember the elliptical pattern I mentioned. The size of this vibration pattern increases as lighter gauge strings are used. Therefore, instruments using extra light gauge strings often require more relief than those using medium gauge strings.
  There is also a need to adjust the relief according to the players style. An aggressive strummer or bluegrass player will often have a heavy handed attack that requires higher action and/or more relief. While a jazz player or gentle fingerpicker may be comfortable with a flat or nearly flat neck. While a flat neck gives the instrument lower and easier action, there are obviously some playing styles that dictate the need for relief and higher action.
• Check your necks relief 

Here is a simple way to check the relief in the neck. With the instrument strung to pitch fret the 6th string on the 1st fret and also at the 12th fret with your other hand.
By fretting a string on the 1st and 12th fret we can then use the fretted string as a type of straight edge. Upward bow or lack thereof now becomes apparent when comparing the straightness of the neck to the "straightedge" string. Now observe the space between the fretted string and the 6th fret. This gap can be measured with a feeler gauge if need be. If there is no gap, 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 the relief.

Adjusting your truss rod

Caution Caution Caution

Stripped or broken truss rod is a very expensive repair, expect to pay $450 and up in most cases. On inexpensive instruments broken truss rods are usually the kiss of death unless the rod can be removed without requiring fingerboard removal. If possible...first practice on a yard sale special! 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. (Normally 5mm for Martin, 5/16" for Gibson) 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 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. Dry threads should be lubricated to prevent seizure. I like to remove truss rod nuts and lube them on older instruments when making adjustment.

Action Truss Rod - The double action truss is capable of adding back bow as well as forward bow (relief). Most instruments only require the ability to counteract the strings tension on the neck, so a single action rod only compresses the neck in a way to bow it backwards. Once they have been loosened it is up to the strings to pull the relief into the neck.
With double action rods it is possible to adjust that relief into the neck even without the strings help. And another word of caution: I can not tell you how many people have gone under the assumption that all they need to do to lower their action is tighten the truss rod. So they start cranking away until the action seems lower or they hear a popping sound and the truss rod nut comes off in their hands. Unfortunately a lack of understanding has led to many mishaps and some insist on over tightening the truss rod as an attempt to lower action, giving no thought to neck angle or saddle height. If you take the time to read my comments on action adjustments you may gain some understanding of how each adjustment affects another.

Truss Rod Adjustment Limitations and Problems: On most newer 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 problem is when we have already arrived at a maximum adjustment. Either the truss rod is completely loose and the instruments still does not have enough relief. Or, the truss rod is as tight as possible and the neck still has too much relief. These situations require fingerboard planeing or refretting

Repairing Broken Truss Rods: A stripped or broken truss rod is the kiss of death for most inexpensive instruments and is usually only considered on instruments of value. In order to replace a broken truss rod the fingerboard must be removed as will any filler material which may be covering the rod.
The procedure can be quite time consuming on some instruments. The finish which is sprayed over the neck and fingerboard contributes to a smooth fluid feel of the neck but also creates a cosmetic problem. Separating the fingerboard from the neck requires that this finish line be "broken" and normally results in some chipping which requires touch up or overspray.
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.
Once the board has been removed it must be clamped perfectly flat to prevent warp or twist and if the instruments fingerboard is old and brittle, removal can prove treacherous. In fact, there are many cases where salvaging the board, binding, frets and inlay far out way the cost of complete board replacement. It simply depends on numerous factors.
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.
Whatever the case, it's now time to dig out the rod. I have encountered rods inlayed in what I can only describe as "cement" which proved to be a real pain to remove. After the truss rod is removed the channel must be cleaned and prepped for the new rod.
When the board is glued back in place you will find that at the very least a fret leveling is usually in order. However, if the current frets are not high enough to allow for the task or the board warrants it, it will be undergoing a refret as well. The board has gone through quite a bit during removal and it is not unusual for the frets to require attention.
With the board in place the finish work can now be done.

Good Luck and if in doubt take your guitar to a professional!!!!!!!