Here's how I do it (a little bit different from what others have posted): I don't pour my lit coals on top of the unlit when I start the WSM. Instead, I pour them in a pile at one side of the charcoal ring. So, while the chimney is lighting, I pour lump into the charcoal ring to about 2/3 full, then push it to one side to make a "hole" to pour the lit coals into. This requires moving and "swishing around" the lump, like Burnt Food Dude described. I end up with a pretty tightly packed load of lump, with an empty space at one side. When the coals in the chimney are ready, I dump them into the hole, giving me a small, hot fire that will gradually spread as the cook progresses.
I don't think you need to pack the lump perfectly (like playing the old computer game Tetris). If you just put your hands on top of the pile, and move them back and forth a little bit, the pieces will settle together well enough to eliminate any big air pockets.