If you have a lot to do, take a look at the fat grass recipe. It's supposed to be really good to revitalize a lawn and I bet on a smaller scale to do patches. http://www.cityline.ca/homeandgarden...goryName=Lawns
Otherwise if you soil is good, you can sprinkle seed on top and then some peat moss (I think that keeps it moist and hides it from the birds). You need to water every day until the seed sprouts to ensure the ground stays moist.
I can tell you (since I just had to go buy my second bag of seed) that it seems that top dressing of peat moss is important. The birds around here got a feed I can tell you.
Oh yes, buy enough seed to throw over your entire lawn because if you buy a seed that is different from the grass you have it will be noticeable. At least getting some mixed into the existing stuff should help prevent that.