I use Schwalbe Marathon Plus Tour (700x35). I've had 3 flats in about 8 months. I prob have around 3k miles on em and they still have plenty of tread. I've heard them described as "boat anchors," but I think they roll better than the stock Conti's. They are kinda pricy at 45-50 bucks each. I recently put them on my newly acquired Surly LHT, so I'll prob get a pair of Schwalbe Durano for my roady bike.

Previous posters are correct. If you ride on the shoulder (like I do) you are much more likely to get a flat. I would ride in the car lane, but most of the year there are a huge number of college students from up north (Chicago), and yeah, I don't wanna die.