They now insist on you drinking quite a lot of water before you donate, as it helps prevent it, and they suggest you do leg exercises during the donation itself, as test helps keep the BP up.
It happens to a lot of people on the first donation (including me - I've now done over 50.) It only happened once since, on a very hot day, in the chippy after... So if it's happened once, it doesn't mean it will happen again. However, I do have a friend who's been asked not to donate, as she was a persistent fainter. So it depends on the individual to some extent.