In case this happens before you make the changes that Schwim listed, or if you decide not to do those changes here's how to get yourself going again.
If it does happen, remove the air cleaner and see how much oil is in the boot, usually you can sop up what's in there with a rag. However if there is a lot remove the hose clamp that holds the boot onto the carb, and pinch the boot to allow a gap to form around the bottom of the boot to allow the oil to drain out.
Remove the spark plug, hold down the kill button-very important, and kick the bike through with the throttle held wide open until no more vapor is seen coming out of the plug hole. Replace the plug with a new one, or use a little gasolline from the tank to clean the old one and the bike should start.
It has also been my observation that if you can place the front wheel slightly higher than the rear so that any remaining oil that has entered the main and pilot air jets to run back into the boot when you are attempting to start the bike.
