I put a piece of black electrical tape on the seat post right where it meets the seat tube, and leave a very short 'tab' of tape - that way when I put it back in after removing it, it always ends up in the exact spot to the mm where I left it (because the little tab prevents it going too far into the tube). It happens to match the colour of the post, so it's not super ugly or noticeable. *Edited to say I'm now seeing the other suggestions re. tape... guess I'm not the only one who does this :)