I find that actually RIDING the things works. I have owned mine now for 17 years and 80,000 miles, and it still has the same pipes fitted as when bought - and they are NOT rotten (in fact two of them are unstamped, possibly fitted by Honda in '69), despite our crap UK climate. Secret seems to be getting them properly warm when used, and storing the bike when not in use in a dry (not damp) garage.
Further to the two stroke reply, it was once suggested to me to add two stroke oil to the petrol at approx. 100:1 mix - enough to cause an oily coating on the inside of the exhaust, without affecting how they run.
Cheers - Chris R.