the spit thing is old... it's only minimally effective for diving/scuba/snorkel masks. generally an anti-fog spray is used (which primarily contains isopropyl alcohol and dish soap or something close to it), or a wipe-on, buff-off anti-fog paste. similar products exist for paintball, some of them are better than others of course... the best thing is a good lens system with dual pane and hydrophobic coating on the inner pane (which of course, you shouldn't touch with anything even lightly abrasive). to maximize the lifespan of thermal lenses you should wash with warm water, not super hot water, because it can promote separation of the lens panes (I've done it on an I4). best thermal lens systems include the Dye I4 and the Empire E-Flex.
PS, if you DO manage to separate your lens panes due to using too hot of water, or a manufacturing defect (fairly common), you can generally keep using the lens as long as you get any moisture out of between the panes, and seal it back up with silicone or an adhesive that won't attack the materials. to get the moisture out, you can use an alternating process of hot and cold air with blowdrier, along with shaking the lens wildly... the process will take about 30 minutes and it is a total pain in the ***. You could also just buy replacement lens