definitely talk to the manager, although i'd wait until after the meal, if you intend on eating there- you don't want anyone spitting in your food!
i once wrote a note to the waitress on the back of the bill, pointing out that as a mother, i go to eat with my family so that i can relax and enjoy my meal. if i have to flag her down for drinks, napkins, etc., then i'm not relaxing. also, we ordered 4 kids meals (sam still doesn't eat enough to warrant buying him a meal- he's barely 2), and she brought only 4 desserts for the kids. i've never had a waiter at the elephant bar do that before- they always bring enough for all the kids. and if she'd expected me to pay for it, she could have at least offered the option- instead i got to deal with a crying baby and asking one of the other kids to please share in exchange for something at home.
anyway, it's important to let the waiter/waitress know why you're leaving a bad tip, if you are, and it's good to let management know because their goal is to have you come back again and again.