This is a simple paper tool to make it easy for you to always know how many hit points you have.
The tool consists of numbered cells, in columnar strips. You'll fold back the cells that represent HP you don't have, whether because you currently never have those HP (they're above your maximum) or because you have temporarily lost them to some sort of injury.
The Hit Point Counter is available as a PDF file.
Owing to the finite bounds of the US Letter paper size, the Counter tops out at 345 hp. If you become tougher than that, you could make a bigger Counter, if you don't mind editing a PostScript file and printing it on larger paper. The key variables to change are
numrows (number of HP per strip) and
hp (total number of HP);
numcolumns is derived from these two by division. Values of
hp that aren't multiples of
numrows will produce less-than-useful Counters.