Rust might be a problem, are you talking stock exhaust tips here, or an aftermarket system? On my TL-S exhaust I use a metal polish called Blue Magic, it works really great, and gets everything off and makes them nice and shiny! I'd try a high quality metal polish, if that doesn't work, I'd say the only other options (besides getting a new exhaust) is sanding them down and getting them refinished. The best way to take care of the rust is to not let it happen in the first place, I usually put the metal polish on mine followed up by a coat of wax about every two weeks, just so they don't start to rust. Once there is rust, it's hard to get it off! If the rust isn't too bad, the metal polish should take it off, just use a couple coats and keep rubbing it in until you see blact soot all over the rag.