How many times have you returned to a project or swatch to pick it up again and not known what hook size you were using? The hook having long since been taken out of the bag to work on something else!
We would love to be super organised people who tag everything they are making, noting pattern, hook size and yarn info on a helpful tag. We’d probably manage to lose the tag! But we have a tip that's easy to make a habit of as it requires no tools, no paper, no pen.
Simply tie knots in the tail end of the yarn to note the hook size. Tie knots close together, one for each mm of the size and then leave a space and tie a knot if its a half mm size.
(If using a hook in a small enough size that there were .25, .50 or a .75 then tie 1 knot in the decimal section for the .25, 2 for the .5 and 3 for the .75)
So simple, so effective. Make it a habit and you'll never wonder what size you used again!