Greg Hinge is a ski technician in Mogul Mouse's ski tuning and repair shop. Hinge had a lot of advice for keeping skis in optimum condition. Unwaxed skis are not just slower- they can also build up a static electric charge which will actually attract more snow to your bases. You don't slide a lot when snow is stuck to your skis. While it would be best to wax your skis every time you go up, waxing them every few trips is enough.
Fluorocarbon wax is great for warmer, wetter snow.
Waxes are now being offered that can be applied to ski like a stick of deodorant. You can use base cleaners that you just spray on and wipe off. I like to iron on hot wax and then scrap it off before it dries. That really cleans the ski. Basically any damage that you think affects your performance you should bring in to be repaired.