I use the S100 Engine Brightener. They sell it at the dealership. I think it's $8.00 for a little can but one can has lasted me almost 2 riding seasons. It comes with a little spray applicator nozzle so it's pretty easy to apply. It works great!
It's probably the same stuff you can use to rejuvinate your BBQ grill.
S100 gets my vote, my 04 Train looks like new. Pigspit is also just as good. I have tried both and would recc both. They sound expensive but they really last, I only re-apply every 3 or 4 washes.