There's a few things you can do...as marblecrusher said, the simplest thing to do is pull the door panels and see where they're hanging up and correct the issue.
There are three other things you can do...one is to replace the felt pieces in the window tracks. The second is to take some grease and grease up the regulator gears and pivot points.
The third is a bit more involved....this would be to pull the motor out and rebuild it. The motor actually comes apart pretty easily...once you get it apart, put some grease in the bearings and the gear assembly. I would suggest reading
this thread for more info.
Hope this helps some...good luck!
