Tire liners are a cheap way to prevent flats. I'd recommend Rhinodillos; They're amazing, >2000 urban miles, no flats on Conti Super Plus's. They come in various sizes and are remarkably cheap for how well they work. I recently got another pair for $12.99 on ebay, free shipping too. I'd stay away from slime liners, absolute trash, Mr. Tuffies are meh... Overall Rhinodillos are fantastic, I've pulled nails, glass, staples, thorns, etc, out of the tires, and no flats ever.

bit of advice with for any liner: take the time to install them properly, line them up right. Improperly installed liners will cause more flats than they would ever prevent.