Eventually, I realised I had a useful tool for this already - my online calendar. I use Google Calendar for this, and it is, of course, very easy to choose a Saturday at the appropriate time of year for pruning each tree or bush. So that's what I've done, setting pruning dates out to mid-2012. If the actual dates don't suit closer to the time, I can move them, and once I've done the actual pruning, I can mark next year's calendar. Apart from anything else, this will give me a record of what I've planted when.
I'm reckoning on doing this with other garden tasks as well - particularly planting. That way, I'll have a record of planting dates without having to mess around with other spreadsheets or external gardening diaries, and can see over time what works well and what didn't.