If something is bent, such as the frame or triple tree, then you may not be able to get it perfectly straight. Something you can try is a shakedown method mentioned by steell a few years back. I don't recall the specifics, but from memory I'll give it a shot... Make sure the handlebar clamp bolts & upper triple tree bolts are tightened correctly, loosen the lower triple tree bolts, loosen the axle nut, then vigorously push down on the handle bars a number of times compressing the forks (you may need a helper to hold the bike steady so that you don't drop it). Afterwards, you can check the upper fork tubes w a piece of glass which should lay flat w no "rock". I don't know how accurate the method is, though, in centering stuff.
