You really just need to learn to slip the clutch. Find the point that the clutch starts to grab, and hold the lever there.. Then apply the gas.
I have a good video from Gary Semics, where he shows how to get a fast MX start, he says, "when the gate drops pin the throttle and use the clutch to deliver the power to the rear wheel."
Or in other words, slip the hell out of the clutch.
Now, I wouldn't do this all the time, but there are going to be times when this is the only way to get going. Like going up a hill after a stall. Find a nice level place, and practice it. You will find it much easier to get up the hills once you have learned to not just dump the clutch.