Start by cleaning the tracks. Don't know what you've got, but ours are brush-lined as weatherstripping. The bottom tracks seem to have collected the dust of ages. If you have glass sliding on metal than perhaps one of the dry spray lubricants would be helpful.
Our sliders are lined at the bottom with a sort of brush-like fibers. I've vacuumed them out and sprayed a silicone spray that dries quickly. That seems to help, but they're still on the stiff side.
The primary reason for windows not sliding is salt encrustation in the track. It does not take much in the way of salt crystals to lock these tight. Easy answer for initial treatment is simply hot water dropped into the track - more difficult in the upper track but same approach. Then clean and spray track with a Teflon spray - I used Dupont Teflon motorcycle chain lube (because I had some and it dries non-sticky!!).
Clean and McLube SailKote, as already suggested. Works great for both our windows and sliding pilothouse doors. We have to reapply around three times per year. It doesn't leave a film like silicone, which is a good thing.