You will see people recommending things like Blue Job, oven cleaner, and other things thought the one that I was most impressed with was the heavy duty magic eraser. It took some elbow grease, but dampen the magic eraser and it'll clean the gunk off the pipe without scouring it to death.

As far as how to stop the melting, good question.  I bought a Harley dyna (as many here do) for my bike and the guy gave me two of the small heat shields that clamp onto the main muffler and that has helped me. Dont get me wrong...It still gets warm on the shield but as long as you're moving air between the shield and muffler (moving...not sitting...) it stays cool enough to not melt things.

This has worked for me every time.
Get a few brand new single edge razors. Using a very low angle, shave off the plastic. Don't flip the blade over or it will scratch. Keep shaving til the blade gets dull, then start with a new one. When most of the stuff is off, use a chrome polish such as SemiChrome and polish the pipe.