Hey Starkler, Dave's suggestion about a tape recorder is by far the cheaper method. Those MP3 player thingos can get quite pricy if you're only using it to record voice messages. Some mobile phones have voice recording like my sister's old LG phone but it doesn't seem to be a common feature so again, you would be buying an expensive item for just one purpose. So go the tape recorder. They still sell them don't they?
I've tried keeping a pen and paper log and it really is very difficult...especially when your red light torch runs out of batteries! GRRR!!!