I started overseeding our pastures and hay fields over the weekend and should finish by the end of the week.
Over the years I have had really good like revitalizing our fields by overseeding in early spring. Some people like to overseed on the snow; I wait until the snow has melted. It's ideal when daytime temperatures get above 32 and the ground is soft (almost muddy) and nighttime temperatures drop below 25 degrees. The colder it gets at night the better because the extreme thawing and freezing pulls the seed into the grown. This process eliminates the need to cover seed or roll your fields.
I've had my best luck overseeding orchard grass, brome, timothy and clover. The seeds vary in size and weight so I buy them separately from a hay seed supplier. Pre-mix hay/pasture seed is OK, but there tends to be more of the grasses I really don't want as well as fillers.
Our guide to pasture management is out and includes more information on overseeding. Copies are just $3.95 for an electronic PDF copy.