Instructions
Study your target and take in all of the details. You may have a good bathroom prank in mind, but if the person is a work associate, then that may not be the best plan to take. Learn what it is he does, what the doesn't like, and even as many details of his daily schedule as you can because timing is paramount when it comes to setting up a prank. For example, if he is single and looking, consider signing him up to a dating site that he would never join.
Get others involved. The best pranks often involve a collaborative effort. This is especially the case when timing is sensitive. In a sense, it's not unlike a surprise party. For example, you may need a friend or two to distract the other person as you finish up the details of the prank.
Embrace simplicity. Most everyone has read of elaborate, multifaceted pranks that required a lot of luck to pull off, but even the simplest of pranks can have just as much impact. For example, have the target think that you're about to do a magic trick. Climb a step ladder to reach the ceiling, and hold a large bowl of water against it. Then, have the person hold a long stick against the water to keep it in place. The "magic" is that you will make the water disappear, when in reality, you will simply walk away. She will have no choice but to get wet and feel ridiculous.
Sell the prank, no matter which one you choose. It's important to avoid smirking or being obviously coy, lest your prank get exposed. This can be difficult because you know the hilarity of the end result, but you should keep a lid on your emotions until the right moment.