A friend sent me this question and I figured I'd see if you guys have any suggestions. He's not a dumb, so I've assumed that he's been tightening everything down fully (but will of course suggest that when I report back).

My crank arm keeps working loose on my non-drive train side, I understand that this means there is damage that is causing the taper on the crank and the taper on the bracket to not engage fully and therefore pedaling works the bolt loose, but is it more likely that the damage is on the crank taper or the bracket taper? Its not a sealed cartridge bracket, its the old style with the ball bearings/cups/spindle.

Crank taper is more likely as that's the softer material

I have the same problem.

It can be caused by using wrong taper crank/bb combination (ie Shimano with Campag, or Suntour with Shimano).

It can be caused by metal combinations (ie titanium bb with steel bolts, alloy cranks).

It can be caused by not having a really good allen key to do up your bolts with (I find the old fashioned Campagnolo bolts you do up with a 15mm spanner tend to hold a crank on that would otherwise come off).

Some people take desperate measures - Loctite on the axle, or the bolt. Personally, I like to know I can also remove my cranks again one day.

don't forget to grease the spindles before you put the cranks on.
sounds like they may not have ever been fully tightened in the first place.