Cali:

I mountain bike all the time and never get flats. Are your tires worn? Check them carefully and make sure there are no cuts, nicks or debris you aren't clearing out. Are you running the tire at proper pressure? Say 35 - 40 #. Too low will cause pinch flats. There are some good tires out there (I use MutantoRaptors and love them especially for silty/sandy conditions like the So Cal Mountains) that contain Kevlar.

Stay away from Slime. OK for road tires but put enough in a mountain bike tire and the tire becomes very heavy and very sluggish. Just too much area to cover and it all piles in one spot (if the turn the wheel it immediately stop where all teh Slime is piled up - half turn, done, no more rotation). I tried it once and never again. Make sure you are using good tubes. Bless them, Performance tubes suck. Try Specialized instead.

It really sounds like you have a small sticker or glass in the tire that you haven't cleared out. When you get a flat, blow up the spent tube and check where the hole is. Find that same spot on the tire and check for debris. Each time you get a flat you should be checking to see if its the same spot. First thing first is probably buying a new tire.