It's usually either rust/crap in the tracks, rusty regulator arms/pivot, or both. Carefully take off the door panel and get a good look at everything. WD40 may help loosen things up initially but it washes away pretty quickly. Get a can of sprayable white lithium grease & use the nozzle extension to get in and lubricate the tracks & the regulator arms/piviot as well as the gear itself and you should see a big difference. When you're at Napa (or wherever) getting the grease you might want to pickup a box of replacement nylon panel clips for the door panel because it's likely that one or more will break, also be careful removing the panel so you don't break the little housings those clips slide into (don't know *how* many of those things I've seen broken)