I've been reading a good bit about this. First I use either one of those foam backed sanding pads (fine grit over 200) like you can get in the paint department at Lowes/Home Depot for tough grime, or a nylon scouring pad like you use for dishes to knock off the large debris (preferred method). Then I use warm water in a spray bottle and a paper towel to wipe the dirty eggs off: A mild detergent can be added in the proper proportions if necessary.
From what I have read (college ag departments etc), it is important to use water that is at least 10 degrees warmer than the egg, but not too hot: This causes the eggshell's pores to contract and push the dirt outward. Immersing eggs in water for any amount of time can allow contaminates to enter the shell.
So far, the best thing I have found for cleaning eggs is making doing what I can to make sure they don't get too dirty in the first place. Most of my hens use the nesting boxes: only a couple of my girls are bad and hide their eggs. I change the litter in their nesting boxes every morning, so I only get an occasional dirty egg, mostly in the afternoon.