Beware that in our zone (mid-atlantic) it's too late to plant grass with much chance of success, unless you can water the heck out of it. If you really want to seed, I'd either a) wait until fall, or b) hire a landscape contractor to hydro-see and thatch it (straw or rolled covering). This way you won't have to water as often, and it's a good bet that it'll take.
If the bare spots are small (~2 feet diameter), you might try those newfangled seed mixtures that are guaranteed to grow. The seed comes mixed with fertilizer and, more importantly, a moisture retention material. I've used this with great success. I think I used the Rebel brand. Expensive, but it works even this time of year. Make sure you match the seed type - using a different type of seed will be noticable when the grass grows in.