I shamelessly stole this idea from the Crave Photography blog. I thought I knew how to do it, and it worked! I used a tripod and posed the kids. I snapped a shot, then yelled, “run!” One kid, chosen in advance, would run out of the frame while the other remained frozen. I snapped another shot. Then I merged them in PSE, and lowered the opacity until I liked it. The kids had a blast!