pedal crank moves and tries to slip off the axle I changed the bottom-bracket in my mountain bike two years ago, the problem was that the bolt on the crank on the non-drive side was continuously getting loose, to the point that I had to stop and tighten it again every mile. 

With a new BB the bike was all right, and it worked fine for almost two years (I wasn't using it that regularly though). Until last week, when it started having the same problem again. 

The BB is a part of the bike that usually lasts forever, so I'm not really convinced about changing it again, because the current one lasted so short, I think there could be some other problem in the bike, that causes the BB to wear faster. But I haven't got a clue about what that could be. 

Does anyone have experience or thoughts about it? 

thanks,
My guess is that by the time the first bottom bracket (crank axle) was replaced the left side pedal arm was also damaged. When you installed the damaged crank arm to the new axle it worked for awhile. You are now at the point that you have a worn arm and a worn crank axle. Replacement of just one of the components will result the premature failure of the other. You need a new bottom bracket and crankset. As this is an expensive repair it may be worth looking for a used assembly that is in good shape. The crank arm bolt should be tightened to about 30-35 ftlbs of torque. This takes quite a bit of effort with the average 6 inch long allen wrench.