jjboods:

700c tires, 35mm wide, with an invert tread. These are a compromise between very narrow, slick tires of a real road bike and fat, knobby tires of a mountain bike; they're much better suited to city streets than typical mountain bike tires would be, and they'll deal better with a little sand or soil on the road (and rough surfaces, etc) than a very narrow, high-pressure tire might. Just make sure you keep them fully inflated (70-80 psi at least) for a smooth ride and to avoid flats.

If the tires lose pressure, you'll have a harder time maintaining your speed and you're much more likely to get a flat.