[willy]

What Frank said is great for a shock conditioning of the engine. But the ideal way is to run it in your fuel all the time. I forget the mix per gallon but it is not expensive and it conditions your fuel, stabilizies it and help get water and crap out of your tank.
The main thing it does is prevent crud in your carbs and literally cleans them as you run your engine along with preventing carbon build up in your engine's cylinders. A yamaha tech at the local marina says engines he rebuilds and works on shows the results everytime. Dramatic difference in the Sea Foam run engines.
;)