I visited a bloke yesterday who has been gold detecting as a hobby for a few months. He showed me a diary that he has set up on an Excel spreadsheet. It's a really well thought out thing. It has columns for recording the amounts of gold found, the amount of hours spent on an outing, GPS/grid references and a comments section where he records such things as weather, terrain, amounts of junk, any other significant finds, even whether insects are a problem. The gold found has a cumulative total that can be referenced to current spot price for stash value. He can also calculate the return on his labor (if any). Maybe a good way to judge the value of your outlays and efforts. If he sells any nuggets he has a seperate section to record this. It has the spot price at time of selling and a column for actual sale price of nuggets compared to spot and the difference. This is easily the best recording system for a detectorist I have seen, but even just keeping a written diary in an exercise book of your detecting adventures is not a bad idea. It might not make sense if you only go out every now and then, but when you have been to many places over a period of time it can help with planning future outings or tactics.
For taking your notes in the field, then to be able to add that to the laptop later in the day. You may find one of these pads useful. Especially on the East Coast where it can be a bit wet. http://www.prospectors.com.au/c-1165-all-weather-writing-paper-rite-in-the-rain.aspx Bit expensive but they are good, get the pen as well IMO. I use both, the pad has grids which helps as well. X Marks the spot
All the best anglers on the coast have diaries, apparently they work. I started one about 20 years ago, it went: "11/02/96 Bright sunny day, high tide, 11:15am, caught nuthin" "23/06/96 Slightly overcast day, high tide 2:34pm, caught nuthin." "6/08/96 Bright sunny day, high tide 4:34am, fucking caught nuthin a turtle-heading again!!" "17/10/96 Who gives a shite what the weather was like? Who gives a toss when the high tide was? What did I catch? You guessed it....nuthin." I don't keep a diary anymore, I buy fish from the wholesaler. oops, edited for swear word.