OK! I just got back from the kitesurf shop, bought myself an ex-demo Liquid Force waist harness for half price ($125 instead of $250), it seems quite comfortable and I was hanging off the rig in the shop, no riding up at all. Also got some nice line from Bunnings for the harness lines, I'd love to use the proper ones but no windsurf shop in my area means no proper gear

Jakub, yes drifting is my forte

Although I've pretty much managed to figure out how to uphaul in between waves, the problem is when the wave hits me and the board stops moving, while I keep going forwards!
The harness should help though.
Thanks again to everyone for the input!