Stop tightening it. Your crank is DONE. Finished. Toast.

If you are having to tighten it after every ride, then the square taper interface is most likely distorted. It oculd be hollowed out or stretched out like the previous posters said. This happened to me once. You are going to have to get a new crank, there is really no 2 ways about it, sorry man...

BUT, if you stop riding it now, you can save your BB spindle. The BB spindle is made of hardened steel while the crank is made of aluminum. The crank will hollow out long before the BB gets rounded off. You can, however, destroy the BB spindle if you keep riding it w/ a messed up crank. That's why I suggest you stop riding this crank, replace the crank, and hopefully you can keep the BB. Good luck.

EDIT: When you get a new crank, make sure that you install it properly. This means using a torque wrench and tightening the crank arm bolts to spec. A good LBS should do it this way.