Okay, I'll say it.
Lube
Nyogel is one of the most popular brands here, especially since Surefire uses it. Currently I use Nyogel 759G on conductive threads and Nyogel 779ZC on o-rings and non-conductive threads. This is the suggested mfr. purpose for them, I believe. These two were originally sold on Lighthound.com and since I bought so much of it I have plenty left. You don't have to do this often, but it is recommended to relube your lights twice a year.
I usually take off the o-rings and rub them with silicone grease (Nyogel and other brands) with my fingers. The plastic toothpick from a Swiss Army Knife tends to work well for slipping off the o-rings. Feel free to wipe off the old lube with a rag and put on just a little bit of lube. You'll soon get the hang of it and figure out exactly how much to use. Smear on only a little dab at first, you just want an extremely thin coating on the threads. I usually work the tailcap back and forth a few times to make sure the lube is evenly distributed.
Cleaning contacts
You should do this before relubing, actually. Wipe down the threads thoroughly, take off all the o-rings that you can get to. Spray or rub De-Oxit/ProGold (available at Radio Shack in pen or spray form... I use both) on all the metal contacts (sometimes includes the threads) and follow the directions. Be very careful not any inside the flashlight bezel or on the inside of the lens. Don't touch the shiny part of a reflector, either. Don't use the spray on the bezel, it is too easy to get inside the lens.
That's it! You can use isopropyl alcohol to clean the exterior of the lens but make sure to use the 70% solution because some lenses do not like the alcohol at all.
Flashlights are mostly very low maintenance -- the Deoxit/Progold is for better contacts/performance which is marginal but it does work wonders on flashlights that mysteriously stop working... like my Minimag which used to flicker all the time. After taking it completely apart and using the Deoxit stuff on all the contacts it worked perfectly. Lubing is a bit of a hassle but it ensures your threads feel nice and smooth (important on twisty lights) and helps to keep your watertight light... watertight.