Practice - practice - and then some more, well, erm, practice..........
Dragging the back knee is a common thing to do - but if you keep doing it, then you'll end up with a nice compression ding on your deck, so better to get out of the habit sooner, rather than later........
I wouldn't necessarily say pracising on your board on the beach is the way forward, as you still might ding it (unless it's a pop-out or swelly, in which case I'm talking poo), but you can either draw a 'board' in the sand and practice - or there is a product which we happen to sell (sorry - blatant plug, but said product is also available in other shops, etc) called the 'Ollypop'.
It's a towel with a surfboard design on it, complete with foot positioning. It was designed by a surfer (and surf coach) and even when you've perfected your pop-up, it's still a useful towel for the beach.......
If you add 'dot com' to my username, then you'll find a website that might be of use..........
