Can you provide what kind of cutting you're doing? The size of your bit has a lot to do with it, so it kind of depends on if you're doing model detail cutting or slab cutting, barrel bit or grinding wheel, serrated wheel or ball bit, etc. If it's detail cutting, sometimes you just have to acknowledge manual cutting with a blade and be patient.
I tend to actually limit plastic cutting to less than 2 or 3 seconds at a time, cleaning off the flash in between. At the end you can take a metal file (or the dremel itself if you have a steady hand) and clean the flash off in one go!
Speed=friction=heat. Not much you can do about that. Filing the excess melted plastic crud and going slow is the only trick I know.
Depending on what kind of plastic you`re trying to work with, some have a tendency to melt faster than others.
I've found that slow speed and a metal cutting disc actually work fairly well, Still get a bit of melting but it cuts more than melts like you you get with the regular cutting disc. The disc I bought is around the same side as the regular cutting disc but all metal with teeth like miniature version of table or circular saw blade.
The metal saw like cutting discs I was mentioning should work perfectly for that since, I believe, it's a relatively hard plastic. You'll probably still have to watch your speed but it should cut cleaner with less melting than the normal cutting discs.
My go to plastic cutting bit, especially when I need to cut curves or discs out of styrene sheet is this high speed steel cutting bit out of one of those little sets. It will melt, but if you're using a flex shaft you can have really good control and it will spin off the melted plastic so you can still cut very well. Now you have to notice the shape of the cutting edges, because I've seen similar sets where the cutting edges are like a symmetrical star shape, not angled like this, and they don't work nearly as well.
I only use the lowest or lowest two speed settings on my Dremel when using it with plastic. Remove the burrs often.
Otherwise I prefer to use razor saws of various shapes and sizes for plastic ... or even a hacksaw blade on a makeshift handle when precision isn't critical.
The Proxxon rotary tools are popular with plastic modellers because they do low speed with high torque. Will overheat and break in short time if you use them on metal though.