It depends on the ride. For anything normal (under 40 miles), just water is fine. When going over that, or in extreme weather, I like to keep one bottle with water, salt and lemon in it. I really don't like sweet drinks and find they make me thirstier, but sometimes I really do need the salt (especially since I sweat like a mofo). I find adding salt directly to the water, with lemon, keeps the bottles easier to clean and goes down easier as well.

I'll also sometimes keep a tube of salty peanuts with me instead.