While they were actually taking it, I just stared out the window and entirely ignored it (it was a VERY interesting brick wall outside). In the end the worst part was sitting up at the end when I was faced with the sight of all the other donors and couldn't ignore it any longer (was very grateful when they let me go down the other end of the lorry to drink tea).
So now I don't have to think about it again for ages *huge sigh of relief*
P.S. It turns out 2 years was a good guess, the last time I gave was May '07. Must try to stick to my original plan of once a year.