the preponderance of plastic BB cups makes me queasy. Here're some musings on the subject:
-Get an all-metal BB set next time. Some of em are dirt-cheap. All the new shimano square taper BB sets seem to be plastic on the left side, which is sad.
-make sure you don't overtighten or cross-thread the bb cup, regardless of material
-someone mentioned loctite. This stuff could end up effing with the plastic.
-if you're BB is exceedingly difficult to install, you may want to get the shell re-tapped.