Getting up and riding was the hardest part of learning to kitesurf for me. It took me about a week of consecutive sessions (and long walks of shame) to get up consistently. I had exactly the same problems you described wassaby.
I'm not a great kitesurfer by any means but the following points might help you out:
1. As you dive the kite to get up, stand up in tandem with the pull in your harness. Don't expect the kite to pull you all the way up to a standing position (like I did

). I think this is a balance thing so it takes practice.
2. When you initially get up, you need to point the board about 95 degrees to the wind and edge a little. This means you don't travel upwind too much or downwind too much either. Too much downwind and the pull of the kite lines decreases because you are travelling with the kite. Too much upwind and the board speed will decrease below planing and drop you into the water.
3. When you start your power dive, make sure you start at 1 o'clock if you are heading to your right or 11 o'clock to your left. Not 12 o'clock. If there is enough wind you want to clip through that top corner of the wind window and end up with your kite at about 10 or 2 o'clock respectively. If the kite ends up to far to the side of the wind window (near 9 or 3 o'clock) you will not have the pull to keep going.
4. Practice and never give up cos it is the best feeling when you get it :)
My two cents...