Today's Ride

So today I went out to ride at a walking track at my old elementary school, conveniently located across the street from my neighborhood. Next to the school is a little park with some curvy sidewalks that I might use to practice turns. The track is a quarter-mile long, and I was able to make it almost halfway around. I could have done better except for one problem. My left foot kept slipping a little bit forward on the pedal, and after a while it would slide off resulting in a UPD. I don’t know if this was a problem from trying a different shoe today or from having crappy pedals on my 20" Sun. Today I wore Adidas Samba indoor soccer shoes, I decided to try them because I had read that most unicyclists like flat shoes. Otherwise I wear Asics Gel-Frantic 2s, I like how the pedal can settle into the groove in front of the shox-like things on the heel. How can I combat this slipping?