Cassie7 at the Halloween Forum brings us a tutorial on creating wounds using liquid latex, oatmeal, and craft paint. Beyond the fact that it uses easily available off-the-shelf materials it's also fast. With a blow dryer to speed drying something like this can be completed in just minutes. It's also a great base technique that can be customized depending on your needs. Change the color scheme from black/red to green/yellow and you have icky pustules. Then add some maggots to really take it over the top.
Oh, and none of this applies if your subject has latex sensitivity. In that case they'll go into anaphylactic shock, swell up like a balloon, and die. A cool effect, to be sure, but you can only really do it once. So make it count!