Start by redoing all of the connections; clean them up and use a conductive electrical paste on all of them.
Tighten all bolts.
If the starter is spinning but the engine is not then your Bendix Spring inside of the starter is probably broken or worn out; either get a rebuild kit for this (NOT SURE IF THEY ARE STILL AVAILABLE) or get a different starter.
If you have things apart and have a piece of BLK battery cable try this, (this one should raise an eye brow), run a new negative cable from the battery ("-" connection) or from the grounding point on the block to one of the starter mounting bolts; again clean connections use paste.
What this does is reduce the path the electricity has to travel getting from the battery to the starter and back to the battery and in turn this will make the starter work more efficiently and last longer.
p.s. While you have the connections all apart, IF you do the connection cleaning thing, and IF the solenoid is separate from the starter, consider the solenoid mounting, clean it and make sure there is a good connection to ground, use HD external tooth star washers.