1987 Chevy Suburban starter

I am having starter trouble. I installed a new starter 4 months ago and I am having to replace it again. It was cracked in the starter housing wear it lines up with the engine. Is there a speacial trick to installing a starter? When I tighten up the bolts the starter seems to back out a little bit. I used shims but it said that you should not use more than four, and four did not seem to help much. Before the starter quit the car was starting just fine.