I use home-made liners with my bikes. Essentially it's black duct-tape folded over, and another layer folder over in the other direction. So one is folded with the sticky sides out to stick to the inside of the tire, the other one is folded sticky side in to go between the sticky duct tape and the inner tube. On my road bikes I only use one layer however. The "grain" of the tape goes in opposite directions when I'm done to help with resistance. I never use slime because I just find that to be more of a pain than it helps.

I haven't done this method to my new mountain bike yet, but I got flats on the first tires it came with within a week, both front and back. My previous mountain bike didn't have a flat this whole year, and my road bike, once again, had a flat right after I got it but after applying my method above, I have avoided flats to date.

I commute on the same areas each day and whenever I get a flat, it's because I took a different route or travelled in a field or something like that. There are three areas in my city where you are guaranteed a flat if you ride near them. I just use common sense and avoid those areas at all costs.