Putting a bumper sticker on someone else's car that says something stupid is an OK prank. But to stick one on that they might not even notice and shows up when lit with headlights from another car, that's waaaayyyy better.
Note: your target needs to have a black or very dark vehicle for this to work.
Step 1: The Secret Sauce
The main ingredient here is retroreflective black 3M adhesive vinyl. In other words, a big black sticker that shines back at you when you shine a light at it. It's awesome stuff, great for putting on your bike when you want to be a safer rider at night without looking like Safety Boy during the day.
Small pieces can be easily found on eBay. I got a 6" x 18" piece for about $10, including shipping. That's enough for 4-6 bumper stickers depending on how tall you want them.
eBay search for reflective tape
Step 2: Make a Message and Cut It Up
First, make a message you think would be funny to see on your friend's car. This can take a while.
OK, you have your bit of funny. What you want to do is block out where you want your message to appear so that you can spray paint the rest of the sticker.
For me, I had it easy because we have an Epilog laser cutter. I made two overlapping pieces so that I quickly spraypaint it in two passes.
For most everyone else, print out your message on paper and cut it most of the way out. Leave little bits so that it's still attached.
Step 3: Get Sticky With It
The best way to use a paper stencil is with spray adhesive. It's awesome stuff. Well worth a can in your arsenal, that's for sure.
Spray the backs of the stencils with a light coat.
Step 4: Use the Stencils
OK, for me with the laser-cut stuff this was just a matter of putting one layer down, spraying paint, waiting for it to dry, and repeating with the other layer.
For the mostly cut out stencil I mentioned before, lay the whole thing down on the vinyl. Now cut the connections with an X-acto knife and pull away the negative. Apply spraypaint.
Step 5: Find Your Target!
OK, the one drawback to this is that your target should have a black or at least very dark vehicle. But if he does, you're in. Place the sticker on it and see how long it lasts!