I tend to use an Excel clone (Sun's Open Office) and if I'm out, make notes of what I eat and drink on my phone calendar. I have a pedometer I wear all day that records the distance covered. I stopped doing it for a while but have started again recently just in case all these wobbles I've been having are food related. It all goes on the spreadsheet so I can track things and see what's happening. It clearly shows that the highest highs and lowest lows have nothing to do with what/when I eat or how much exercise I'm getting, ay least when compared to my original diary. I seem to be at my highest in late morning and my lowest in the middle of the night and I'm currently veering between the low 3s and the high teens, regularly hitting 17 or 18. My doctor is fascinated by it and reckons it could be a very valuable thing to take with me when I see the consultant.
My phone and laptop talk to one another so I can transfer data really easily which saves a lot of time. I don't tend to use actual pen and paper as I'm always losing such things.