I always have trouble remembering what the tank readings were a week ago. Especially when I am medicating and salting or whatnot. I saw the tip on keeping a notebook by your tank and thought it was a good idea. I tried it. I lost the notebook. So I figured if I have the stats right on my tank I can't lose them!
I bought a small 2 inch post-it note cube. With a sharpie fabric marker (water and run proof) I write down all my markings. Then I stick that to the back of my filter. No one but me can see it and it's right there every week! I like to keep two or three stickies on top of each other because then I can quickly look back at the last few water changes.
Here is a picture so you can see how inconspicuous it is.
You can also write things on stickies like feeding schedule or days that you have to do water changes, rinse the media, that sort of thing. I love using sticky notes because they are so convenient! I like to be able to have all the info I need right there *on* my tank!