Fishboy,
Are you going to broadcast the seed? If so, and you have a heavy thatch layer on the field from mowing, then expect fairly poor results. If you have some bare ground exposed, then it could be alright. Don't know what type of equipment you have, but if you have a tractor that is big enough and that has hydraulic remotes, a no-till drill would be the way to go. You can drill the clover/wheat/rye all at the same time, and the drill has coulters (trash plows) to cut through a heavy thatch layer so you'll get seed to soil contact.
The best time to overseed clover by broadcast methods would be in the late Winter.