You may already know this stuff, so apologies if its a bit basic. I found three tips on popping up that worked for me (in addition to the stuff Jaffa mentioned, which is the key):
1. use your toes to help (see the video at the end, the guy does this). You may already do this without realising. Probably not the technique to rely on if you are intending to shortboard at some future stage, but it makes it so much easier on a long board.
2. make sure you have your hands down near the bottom of your rib cage, to maximise the height you can push your shoulders/hips up. If your hands are up around or above your shoulders, they are too far forward. I found this was a problem for me - after paddling I just brought my hands 'straight in' at shoulder level.
3. 'commit' - don't be hesitant about popping up, you need to really commit to doing it. Its like riding a bike - going slower because you think you might fall actually makes riding harder, not easier.
Oh, keep your head up and looking forward. If you look down at your feet it actually makes the pop up harder because the weight of your head is 'in the way' and you are unbalanced whe you stand (okay, one last tip - when you stand you should be as low as possible. This makes it easier than popping up and trying to stand tall at the same time - again see the video)
I found the 'aussie sprinter' method described at the end of this video was really useful when I started off. I'm not sure about the 'knee technique' described in the middle as a long term option, but if you are going from knee paddling to standing, as jaffa suggested, then I would guess this is the technique to follow.
http://www.youtube.com/watch?v=XoH8xWlPNeg