I've heard of a lot of people using dielectric grease and I tried it but I don't really know how well it works. I've used Mother's Rubber/Vinyl/Plastic protectant. However, I think I'll give the GM stuff a shot next time.
I have used the Auto Glym protectant too. Still have some in the bottle. Probably any product will be a whole lot better than nothing at all. I do all the rubbers a couple or three times over the summer and last thing in the fall. Not that big of a job but rubbers are about the first visible parts that do deteriorate on a car, from my experience.