"Never rub dry dirt or dust off the surface, because they may contain miniscule quartz particles that are much harder than glass and that can scratch the latter nearly as badly as diamonds can. First gently whisk the dust off with a very soft brush before breathing on the surface and then using a soft cloth, preferably the B+W “Photoclear” microfiber cleaning cloth and extremely gentle pressure to wipe the surface clean. When the dirt is very hard to remove, a bit of saliva is the best solution – always available, guaranteed free of abrasive materials, oil and aggressive chemical substances. Never use a paper tissue for cleaning because it leaves too much lint, and paper fibers cling to glass because of electrostatic charges." This is a quote lifted straight from the Schneider Filter Manuel.
So spit is perfectly OK to use a lens cleaning fluid.
However I strongly disagree with what this Manuel, and others say, about the use of lens cleaning tissues. I have been using nothing but lens cleaning tissues for 30 years. The great advantage of tissues is that they are disposable. The fear of any cloth is that they will collect dirt particles which you will unknowingly rub into your lens. Schneider only complaint with tissues is that they will leave lint on your lens. I can assure anybody that this is just not true - I have never had lint left from a tissue, and that's cleaning lenses several times a day for 30 years. It is more likely to happen with any cloth. All the great lens and camera manufacturers make lens tissues, Kodak, Lee, Tiffen, Rossco. Most professionals that I work with use tissues. Our local pro fujinon repairer uses tissues. Tissues are just as soft as any cloth on the glass as well - that's my opinion.
Using a UV filter for protection. Remember every bit of extra glass on your lens - no matter how good it is - will add to flare, kicks of light etc. For my 35mm work I leave the UV filter on, as it is impracticle to take it off for every shot. It is important to me to protect the front element. However for my LF work I unscrew the protective UV filter just before I take the shot (unless I am using other filters - so I have removed it before hand.)