Every water change, I always used a citrus based cleaner to clean it down to just past the water line. Stubborn stuff might need a magic eraser, like poolguy mentioned. After cleaning, I'd rinse it all down and wax (with hot tub wax) the tub down to just below the water line as well. The wax really kept junk from sticking too much between changes. The cleaner and wax I used were Leisure Time brand because that's what the local store carried. I imagine there are plenty of options out there. Just make sure whatever it is, it's safe to be using in a hot tub. Because,try as you might, you won't get it all rinsed and drained out... Don't want to be bathing in stuff that will hurt you!
Also, they make foam floaters that float on the surface and catch some of the gunk that eventually finds it's way to the sides. You just take them out and rinse occasionally.