Paper napkins. Anything to just record it so you can get it into the computer later. Sometimes you'll have a big stack of napkins at the end of the shift that need to be entered. Write down the time and whatever numbers you need to know. Really!
I 2nd the napkins or paper towels. I often have a stack of them when my patient is on CVVH, because I always try to have a nice little grid on my cheat sheet, but I forget to bring that into the room and have to write it down on whatever I can find!