Resetting fence posts in very wet conditions
Are you in the Twin Cities? We have lots of clay here. Clay never really dries out ;)

Looking at the concrete, notice how it's flared at the top, this is a nice 'handle' for the frost to grab on to and push up. Ideally, your flare would be the complete opposite...wide at the base of the hole, narrow at the top.

If it's really heavy clay, you may want to forgo concrete completely and instead use compacted crushed rock. 

If you stick with concrete, be sure to dig deeper than you need to and put gravel at the bottom of the hole for drainage before putting in the post and concerete. Also be sure to get at least a good 2' of the post into the ground.