I find if I shake it as I add it from a larger plastic bowl (I use gladware bowls) it dissolves more...tries to clump less.
almost, but not quite, sifting it as you add it.
I had this happen as well until I changed when I add the DME. Kind of along the same lines of what bradsul said I add the DME after pulling any steeping grains I use and before I crank up the heat to bring the water to a boil. Seems to me that the lower amount of steam coming off the water reduces the clumping.