As the board's resident expert (lol) on fogging:
Bebo, most defoggers act by reducing the surface tension of water. Surface tension is what causes water to bead up on surfaces into droplets. Those droplets act like miniature prisms and scatter light randomly and voila! you get that white haze we call fog.
If you reduce the surface tension of the water, it spreads out. And if it spreads evenly, then you can see right through it. So most defogging solutions consist of some compound(s) that absorb the water and lower its surface tension. But the trick is to get this compound to coat the surface evenly and for it to stay in place as it goes through the wet/dry cycles.
So if spit, detergent, potato juice, and anti-fogs all work to reduce surface tension, why doesn't your buddies recommendation work for you? First, your mask is different and the glass may have a different composition or coating. It also may be older or newer and the surface may not be the same because of how you wash it, etc. Second, you probably perspire differently than your friend. So you may overwhelm your spit's capabilities to defog. In my case, spit works fine about 90% of the time. Last, conditions can overwhelm an anti-fog. It may work great when you are cruising with the water temp at 68 degrees. Then fail when you start working hard or the temperature cools.
Bottom line? You may have to do some experimenting with the anti-fogs that are out there. I read a review of anti-fogs designed for Scuba that is a bit old, you can find it through Google with a bit of searching.
Personally I am a bit skeptical of using toothpaste to rub down a lens. Virtually all of them have some sort of abrasive and with enough brushing, you will introduce microscratches into your lens. Or you may remove or scratch the coating the manufacturer applied.
Here is what HydroOptix says, "The mask's lens surface(s) have a hard coat for protection against abrasions and scratches, however such damage to lens surface(s) is not covered under warranty. Be sure in your handling of your MEGA mask to follow the directions that were included with your mask; scratched or otherwise damaged masks cannot be accepted for full refund."
I would follow manufacturers instructions which come with your mask and then try a few of your friends favorite anti-fogs until you find one that works with your combination of conditions. Another good idea is to test one half of your mask by applying the new anti-fog to only one side, so you can compare how it performs. When I test, I coat the right with one anti-fog, the left with another, and leave a one inch strip without any anti-fog in the center.