I've been using Yamaha RingFree in my 4Stroke Yamaha. I've been told by multiple service techs that when they have to tear down a motor they can tell the ones that had used RingFree to prevent carbon and such and they recommend it. It's not cheap but I guess it's cheaper than major motor issues
I use non ethanol gas as much as possible. Since 2005, I have owned 2 Yamaha motors. The first one (2 stroke, 150) had 900 hours on it and the second one (4 stroke, 150) now has 700 hours on it. Neither ever saw a mechanic or boat shop. I performed 100 hour services myself and winterized the first motor myself. I don't winterize my current motor.
In terms of my driving habits, I drive like a bat out of hell.