I add about a gallon of hot water, approximately 10 degrees above my desired strike temperature. I then cover the cooler, and let it sit about 15 minutes. By then, the water is at my strike temperature, and the MLT is preheated. I start adding grains, stirring well so I break up any doughballs, then add more water and then more grain. I stir during this time, and break up any doughballs. Then, I check the temperature in several different places.
This seems to work well for me. I've never had a stuck mash or sparge, and my temperature stays within .5 of a degree of the mash temperature over an hour.