Summarize:

If you want your bruise to look real, you've got to pick a good spot for it. Some places don't actually bruise that much, so you've got to pick a good fleshy place on your body, or a place where you might be injured. The best places for fake bruises are usually:  Your forearm. Anywhere on your leg. Your forehead. Your chest or shoulder. Don't put a fake bruise near or around your eyes. Real looking bruise in place? Check. Unsuspecting friend in sight? Check. Wait for a good moment, when you're both doing something physical or playing in some way, then fake an injury. Wait until your friend comes close and then grab the area with the fake bruise suddenly and start hollering.  Really play it up. Pretend that you're in pain and look angry. It's a good way to build some sympathy or freak out the victim of your prank. If you don't want to go through all the acting trouble, you could always just whip out your bruise and say something like:  "You should see the other person." "I got this working on an oil rig." "I fell off my dad's motorcycle."
Pick a good spot. Fake an injury.