Just lower your throttle and it will reach a point where it will kick the other way and run correctly. You may have to jazz it up and down a little.

Putting your hand anywhere near the running prop is not smart, even though you see other guys doing it. One slip and there's blood everywhere. BAAAD HABIT!!!

If your engine tends to start backwards a lot, flip it backwards and it will start in the right direction. After choking my engines most will start with a gentle bump backwards.