I've previously mentioned tracking damage with tally marks on scratch paper instead of erasing and recalculating hit points after every wound. As helpful as that is for PCs, it's even better for the DM, especially with a couple extra tricks.
Instead of recording hit points in modules and DM notes beforehand, just note the hit dice, which you need for attack and save purposes anyways. For a single monster with just a few hit dice, roll the dice the first time the PCs score a hit. If the PC does enough damage on the first hit to kill the monster, you've just avoided some writing and erasing. Otherwise, remove one or more dice equal to the damage done and let the remaining dice sit to one side; as the PCs do more damage, remove more dice until the creature is dead. If the party leaves without killing the creature, write down the remaining hit points; use that if the party encounters the monster again that same day, or add 1d6 hit points per two days between encounters.
For more than one creature, roll extra damage for the first one or two hits per creature, to emulate variable hit points. For creatures of up to 8 HD, roll half their HD as extra damage on the first hit; for 9+ HD creatures, roll one-fourth their HD as extra damage on the first and second hits. Mark twelve tally marks per line on your scratch paper (or more, if using 8-sided HD or higher.) When you've filled up a line, you've "killed" 2 HD worth of creature.