Without a doubt, fall is the best time to start clover. More rain, and less weed competition. You can do some annual clovers in the spring with some success, but for best results I would do it in the fall with a good cereal grain nurse crop.
Like wyldwulf says, you'll be happiest with the results from a fall planted clover crop. If you're looking to put in a perennial white clover, now is a good time to get a soil test done, add whatever lime & amendments are required and that will get you well prepared for the fall.
You'll also be able to start some weed control, now too.
Another problem with a late winter/spring clover planting is the roots don't always have a chance to fully develop until it gets to hot/dry which will dry the root and kill the plant. If your plot has a good amount of shade then the clover would have a better chance at survival.
Fall would be best like Canuck5 and Wyldwulf stated.
I learned the hard way in 2010 that the best thing to do is get your soil tested and amend your soil before you ever put a seed down. If you get your PH, N, P, & K right, now, you will be all set to get your clover seed down in the fall with enough growing to hunt over for deer season.
Also, another lesson I learned was that you need a nurse crop, like a grain, to hold the soil from eroding while you wait for your clover to fill in. I use 60ibs of Cereal Rye seed per acre to serve that purpose. The rye also gives the deer etc. something to each on while the clover is getting established.
Doing it the right way, made 2011 a grew year for my plots, and I am halfway to my knees in clover and up to my waste in rye as we speak.
Durana is the most aggressive clover I have used so far.
All good advice. The only other thing I would add is to invest the time to prep your site. Since it's gonna be there for awhile, drag it till it looks like a ballfield before planting. I have one clover plot that I could not get my 10ft drag to because of the narrow trail. I regret not making something smaller for my ATV and getting the job done right