If you are unstable, remember to put slightly more weight on the leeward side of the board. That gets the board to push against the mast and stabilize things for you. Just be ready to adjust when the sail is lifted.
Many thanks for your advice guys, it's all very useful. O2bnme, did you mean "windward" side, rather than "leeward" side? When uphauling, the board is windward, the sail is leeward, if I want the board to push against the mast, I should put more weight on the windward side, isn't it?