This works for me.
Draw an outline of the plot on a piece of paper. Scribble your planting areas on it until you're happy with the basic layout. Transfer neatly on to a master copy, photocopy and keep that. Use a copy for each year, fill in what you are going to grow and hang on shed wall at allotment to refer to. I've got 2012 and 2011 side by side and very helpful for rotation.
Also keep a nice fat notebook at the allotment with a page for each crop you are growing and note everything you do on it - manure, compost, sowing dates etc, etc. This is brilliant the following year as a reminder of what worked, what didn't........
Much as I love computers having a hard copy where you are actually working is brilliant even though it all gets a bit scruffy and scribbled over as I change my mind.