How do you prevent flats on your mountain bike? Mr Tuffy, Slime, Combo of both?

There is no way to completely eliminate flats. Mountain bikes with 24" wheels don't give you a wide choice in tires, 26" is by far the most common wheel size for mountain bikes. Because of that, most of the tires you will find won't be any better that what was on the bike when you bought it. Tire liners might help, but one of the biggest causes of flats in low tire pressure which results in a lot of pinch flats. If you keep your tires at their ideal pressure you will eliminate a bout half of your flats. Even tire liners and heavy duty tubes won't prevent pinch flats.