My main problem was not using my body weight to shift the bike. I'm no master at wheelies but I only got it up about as much as you and now I'm learning to keep it up a little longer (and getting it higher).
At about 7-8mph, lean down and compress the forks, pulling up and leaning back in one motion when they start to rebound, drop clutch and open throttle at the same time. Pull clutch in as soon as you got it up more than last time. Eventually try to work your back brake in so you don't need the clutch to come back down :)