Check out the Avery label program, you can buy a wide variety of labels and the program is extremely easy to use - and cheap.
I use the 2"x4" label, have it pre-formatted with drop info columns and simply insert the drops and/or come-ups into the saved template. I also make a smaller sized label to put onto the front bell of my scope, put the big one on the left side of my rifle. Then I cut a piece of transparent shipping tape to fit over the label after it is on the rifle or scope, round the corners with a scissors, and put it over the label so it becomes waterproof. I have also simply put a bunch of strips of Scotch tape over each label to waterproof it but the big wide one-piece shipping tape is better.
I also carry the same drop label on a card in the Eagle stock pack. We cut large milk containers to get tough, flat pieces of plastic and stick the drop chart label on them, shape them to the Eagle stock pack pocket.
Also carry a card in my shirt pocket - that gives me three of the **** things so I should be able to get numbers fairly quick.
I could take pictures of all this if you guys wish, but do not have a clue how to post them.