Simple adjustments to improve the speed and comfort of your playing
WARNING/DISCLAIMER: If you in any way doubt your abilities DO NOT attempt to adjust your instrument without professional assistance. It is possible to snap the neck of your guitar by over-tightening the truss-rod. Take responsibility for your own instrument; I accept no liability.
With that out of the way, on to the good stuff;
When you purchase any instrument, you have to bear in mind the fact that they are not set-up for you as an individual.
Many things will affect the way in which you set up an instrument; your playing style (jazz, rock, classical), whether you are standing or sitting, even the size of your body and hands. Setting a bass up to suit you gives the ability to play with precision and ease compared to a set up which causes too much tension in your hands, such as with the strings too high.
Maybe you've noticed that your fretting hand gets tired more quickly than you think it should? You might also have noticed a buzzing in certain places on the fret-board. Conversely you may have noticed that bending the strings is difficult and the bass doesn't sound as "full" or "rich" as you feel it should. All of these issues may be solved very simply with a minimum of time and effort so that you can get back to doing what you love; playing the instrument!
The Truss Rod
The truss rod is a long piece of metal which runs through the neck of a bass from the head down to the body. On your bass it might be visible, or it might be covered by a panel which requires a small amount of unscrewing to remove. It's purpose is to alter the curvature, or bend, of the neck in relation to the strings. Bend the neck away from the strings and there is more space between them (the action is higher). Bend the neck towards the strings lowers the action. The bend needs to be balanced with your personal needs and the needs of the bass to enable smooth, relaxed playing,
Once you have unscrewed the cover, you will notice a slot for a size four Allan key (also called a hex key due its hexagonal shape).
At this point you might want to loosen the A and D strings just so that you can move them out of the way a little bit.
Take your bass and hold it so that the body is by your feet with the strings up. You should be looking down the neck of the bass and should be able to see in to the Allan key slot. Now, depending on whether or not you want to loosen or tighten the truss rod you have two options.
Loosening the Truss Rod
Loosening the truss rod increases the bend in the neck away from the strings. If your strings are too close to the fret-board and are buzzing this may be an option for you. Alternatively if you want a deeper sound and more flexibility in string-bends you may choose to loosen the truss rod.
With the bass positioned as above, insert the key and make a slow quarter turn to the left (anti-clockwise). Now check the relief - if you are happy with it then stop. If not, you may make another quarter turn. I do not recommend any more than a half turn total and do not be surprised if the next day your bass doesn't seem that different - the wood and structure of the bass all need time to "settle" in to the changes, which is why it is so important to do these very minimally and slowly. To repeat; no more than a total of a half turn in either direction per day.
Don't come crying to me if you turn it too far and ruin your bass!
Tightening the Truss Rod
All of the above rules apply to tightening the truss rod. And are potentially even more important! Tightening the truss rod "flattens" the neck, bringing the bend towards the strings. This can allow for a lower action and smoother playing and accuracy with fretting.
Again, position the bass as mentioned. Insert the key and make a slow quarter turn to the right. Now check the strings - if they look fine then stop! Once more; no more than a total of a half turn in either direction per day.
With your adjustments made you can now replace the truss rod cover if there was one and get back to playing. Give your bass a little while to settle like I mentioned, and give yourself a little while to get used to the new playing mechanics.
With the truss rod adjusted to your liking and your strings playing without buzz you can turn your attention to the bridge and bridge saddles. These are normally found at the body of the bass and are the bits which hold your strings in place. Typically the saddles can be raised or lowered with a 1.5mm Allan key. These are fairly simple adjustements to make which can improve the action to your liking.
To Sum Up
You now have enough information to make an informed choice; if you feel confident, you can go ahead and make the changes you desire - just don't blame me if you don't use common sense and it goes wrong. If you feel less confident, feel free to do some more research or get a professional to set your bass up for you. I can't stress enough that the changes should be very minimal - that is really all it takes to notice a change either for better or worse. This is also something that should not take up a lot of your time, and if you can't get that "perfect sound" my advice would be to worry less about the bass, and instead concentrate on improving your playing. A bad instrument can still sound amazing in the hands of an experienced and knowledgable musician!