April Fool's Day is an important holiday that is our duty to observe and engage in fully. Designing and executing a successful and distinctive April Fool's Day prank is not something one just does willy-nilly the night before -- sometimes you plan them as much as TWO nights before! Here are my guidelines for pulling off a high-quality April Fool's Day experience:
1) It can't be expected - at all. If someone is at all suspicious of you and is aware of the possibility that a prank may be forthcoming, you have lost a large part of your potential to be successful. Not only will their senses be heightened, but any surprise you do create will be significantly reduced by their cathartic experience of "I was expecting this, now it has happened, time to move on."
2) It must be plausible. There's certainly something to be said for elaborate pranks that attempt to convince the target that they are experiencing some ridiculous deviation from the realm of their normal lives (e.g. being abducted by aliens, losing a calendar day, etc.), but these are not appropriate for April Fool's Day, in my humble opinion. A proper prank must be surprising and even highly unlikely, but still plausible.
3) It can't depend on weak helpers. Far too many pranks go awry when the prankster enlists the help of seemingly committed assistants who, in the end, crack under pressure or otherwise give away the prank. If there's even the slightest chance that your cohorts won't be able to keep a straight face and/or go through with the dirty work, relegate them to behind-the-scenes work or don't involve them at all. A good qualification test is some sort of staring contest.
4) Avoid permanent damage. Targets will be able to enjoy the humor of the situation more quickly if the prank does not incur permanent physical or psychological damage to their person or property. This indirectly increases your enjoyment, since you spend less time dealing with fallout.
I hope you enjoy the holiday, and perform and/or are subject to some excellent pranks.