I live in CO where the Earth is carpeted with goat-heads. For those of you who remain blissfully ignorant, goat-heads are essentially not-so-small stickers capable of easily penetrating bike tires/tubes. They are seeds with multiple stickers, and they grow such that a sticker is always facing up, like a natural caltrop. The evolutionary idea is the seeds hitch a ride on passing critters. They'll get you even if you don't ride in dirt. Nasty little things.
I've always run slime in my tubes as a result. When I got my new bike, one down-side was that pre-slimed tubes don't come in my 700x42 size, and you can't add slime to most Presta-valved tubes. Advantage: shrader-equipped MTB!
After 4 flats in a little over a week, I got online and found some tubes made by Michelin which incorporate a 'slime-like' sealant and are built a bit heavier. Since installing them, I've had no flats in something like 500mi. It's worth mentioning that with every prior flat, I went over the tire very carefully, using touch and sight, to be sure I didn't have a flat-maker stuck in it somewhere.
Slime and its competitors often get a bad rap for being heavy and therefore slow. I find it hurts my average speed much more to be eternally changing flats on the side of the road, but that's just me. If anyone is interested in the Michelin tubes' specs, reply and I'll be glad to post 'em this evening after I get home. I think I spent $30 with shipping for 3 of them. I'm thrilled and hope they will keep up the great performance they've delivered so far!