Spit is all you will ever need.
The only reason spit doesn't work sometimes is when it is used on a mask that hasn't been cleaned properly when new.
Some masks are relatively clean from new, others require proper preparation. From my experience Oceanic and Tusa masks require only minimal cleaning, masks at the cheaper end of the market tend to require more. That's not to say cheap masks are bad. A cheap mask that fits is worth its weight in gold, an expensive mask that leaks is worth its weight in saliva. A mask-sealing solution that you put on the skirt of the mask is just another means to part you with money; perhaps to enable the LDS to sell you a high-end mask that doesn't fit and some sealant rather than a mask that's cheap and fits properly. The strap has no role in ensuring a proper seal, if you have to have a really tight strap to avoid leaks then your mask doesn't fit and you should get one that does.
You can prepare a mask for use by:
Cleaning throroughly with cheap white toothpaste (white rather than gel because the white has more abrasives). Don't forget to clean both lens and skirt. Remember you're also trying to remove the release agent used during the process of moulding the skirt. If you don't get that off the skirt too it'll just transfer to the lens again.
Running flame over the lens on the inside e.g. a lighter flame. This works but, since you can't use it on the skirt you won't get rid of the release agent.
Put the whole mask in with the dishes in your dishwasher for a cycle.
After that, spit will work fine. Saliva is all you need, you don't need to grunt up mucus from the depths. Eventually you will develop a Pavlov's reaction and start to salivate at the sight (or even the thought) of a mask. One tip though: never eat a Snickers just prior to your dive otherwise it will be significantly more chocolatey and nutty than you might have preferred.