Spray deoxit on the pins and then push the tube in and out a couple of times. If it's REALLY bad you may need to replace the socket. Also, your sockets may need to be retensioned... not sure how to do that though.
That answer is pretty simple....electrical contact cleaner
CRC electrical grade contact cleaner 2000 is what I use. You should be able to get it at about any hardware store or electronics store. Just make sure the amp is off and unplugged when you use it. It is flammable and will easily ignite. Just spray it in the socket, and it will evaporate. You could also take a pipe cleaner and stick it in the pin grabbers.
I'd also check the grabbers and make sure they aren't spread out too far. If need be, take a pair of needle-nosed pliers and squeeze them back together very carefully.