Have you ever have a Prank fall flat?  Boy, I have.  

Lets face it, our RC Croc Head isn't going to be funny in a bathtub, and the Nail in the finger prank isn't going to leave them laughing in a Hospital...  Now, I like a long sophisticated build up for my pranks, but to be honest, even a pie in the face works, if you remember the rules of comedy

There is an episode of The Big Bang Theory (The Good Guy Fluctuation) where Sheldon is trying to get back at the guys for a prank, but they keep being turned around on him:

• Sheldon booby traps the mailbox, but Leonard turns it around and gets him. (pay attention to your surroundings, control the environment is possible).
  
  
• Sheldon puts a snake in Raj's desk, but Raj doesn't react scared, he likes snakes. (know your audience, reaction is everything, pulling a prank on someone who doesn't react isn't funny).  This is why my oldest son always brings homes socks with holes in them, he knows his Mom goes wild.  Simple prank, but it works, because he knows his audience.
  
  
• In an attempt to get Howard, Sheldon tries to shock Howard with a joy buzzer.  Maybe a Shocking Lighter would have worked better, but Howard and Bernadette turn it around by a fake condition, and forcing him to jab an (fake) adrenaline needle into Howards heart, and in the end Sheldon shocks himself (don't become the victim of your own prank!)
  
  
• What does work is when he patiently waits in the couch, dressed as a zombie, and pops out when Leonard if completely distracted realized he he lost a perfect potential girlfriend for Pryia, who betrayed him (patience and timing is key).

I'm not saying you should wait until your best friend is miserable to scare him, but the point is you need to pay attention to the environment, audience, and your timing.  Done right, even the silliest prank can be hysterical.

Have a Pranky Day!!