Originally Posted by halfbiked
It may be different with the older cranks, but when I took my tapered-square cranks (roughly from 1992) in for the same problem, the dude said that once they start wearing, you pretty much have to replace them - i.e. just tightening them is a very short-term solution. You may need to find some replacement crank arms.
They don't realy 'wear',but if you ride them with arm loose on the spindle,it buggers the arm to spndle interface.. Keep em tight.