The Basics of Brake Lever Setup
Setting up your brake levers is fairly simple. All you have to do is slide them onto the handlebar and then clamp them down.
But when you do this, there are two things to keep in mind...
Hand and Lever Position
First, you want to position your levers so that you can easily reach them with your fingers while keeping your hands in a comfortable position.
This means that you will move your brake levers so that they match the angle of your hands and arms when riding comfortably in the attack position. Do not adjust your body to make up for poorly positioned brake levers!
But equally important is...
There is a certain tightness that should be used when tightening down brake levers, especially the levers used on mountain bikes and BMX bikes. (It has nothing to do with torqe, but you should still keep that in mind when dealing with fragile parts.)
The key is NOT to tighten your levers so that they are locked to the bar, immobile. The entire lever should be able to move slightly, so that they will not break in a crash.
See, your brake levers stick out. If you wreck, there's a good chance your brake levers will get hit. If they are locked in place, it's likely they'll snap off. Not only will you need to buy new levers, but you'll need to walk your bike home (riding without brakes would be a bad idea!)
But if your levers are lightly tightened against the bar, they'll stay in place during normal use, but if they hit the ground, they'll slide back on the bar instead of snapping.