Do a search on window problems. Basically you have to put some silicone on the rubber rails. Personally I use silicone grease for the rails, and every once in a while I open the door panel and use lithium grease on the mechanism itself. Some people from this forum use silicone spray with good results.

I had once to replace the driver's window regulator - The mechanism was fine but the wire that moves everything inside was a mess, probably because of the lack of grease was putting too much stress on it.

Make sure you do the proper maintenance soon before you end up paying $120+ for a new regulator.