Mix some glue and water, and put some red food coloring and ketchup, its REALLY messy on people, but if you mix it JUST right (its hard to do) it drips down slowly and leaves like "blood" so its really cool. EDIT: (Cooler, More Complex Recepies: http://www.halloween-website.com/fake_blood.htm)
I'd go with ketchup, its the easiest...xD but you could just create a thick liquid and add red food coloring.