Opti-Coat is a good option to protect, but doesn't actually clean the tips for you
I really like s100 polishing soap. makes the task of cleaning exhaust tips slightly less miserable. Works well and its pretty cheap also
http://www.autogeek.net/s10010.html
If really bad I start off with APC and a wheel brush. then move onto the s100 soap. If you stay on top of it regularly, just use the polishing soap
I typically will use this during the wash on the tips. If still grimey after, I may polish whats leftover with poorboys pro polish or klasse all in one