you probably want to go a little faster. Mine does this too if I'm leaning forward too much or not going fast enough. i try to run my bike up to around 10mph or so lean back a bit to get some weight over the rear and then pop the clutch to get her started.
good advice. if that doesn't do the trick, and you feel that it's skidding way more than it should, it's possible the clutch is too tight. you can loosen the pressure plate nut a little.