One tiny tip I have is to use underscores in ids and hyphens in classes. That way if I see $(".things-to-find") the fact that "thing-to-find" is not recognizable as a javascript variable gives that much more of an indication that it will be (relatively) expensive. Likewise #doodad_im_styling looks bad in css files since it doesn't match anything else.

Granted I violate both of these principles, when appropriate, but they're there to keep the average case in line.