You're probably dropping your rpms too low for the bike to stay on in gear. Engage the clutch if your speed is anything close to a stop, especially if you're in a higher gear. Also, make sure your idle speed in not set too low.
I adjust mine by ear. However, the correct way is to use a tach and set it to specs. The adjuster is behind your fuel shut off. Turning it clockwise will increase your idle, counterclockwise will decrease it.