That happened all the time back when these things were new, too. Was a common problem on some Macintosh computers as well. I remember all sorts of kludge/fixes that were done to keep the RAM in - toothpicks wedged in just right, bits of plastic or wood between the SIMMs and the whole bundle tightly wrapped with rubber bands, hot glue, etc.
Obviously, the best thing to do is just replace the sockets. I know that at least a few years ago you could buy them new from Jameco (metal clip version). They're not terribly hard to replace.
-Ian