To give your joke a higher chance at success you want to
- Involve multiple people if possible
- Cause a "mishap" as opposed to a "tragedy"
- Ensure that you're in a non-threatening, relaxed environment
But at the end of the day, you need to use your own judgement and social/emotional acumen to determine whether your joke will be well-received.
I'm going to attempt to dissect this by looking at the psychology behind jokes. Specifically, what will give your joke a better chance at being well received?
Humor in Groups
Robert Provine wrote a nice article on the subject of laughter. He finds that humans are almost 30 times more likely to laugh when in a group as opposed to alone. He sums this up nicely by saying
The necessary stimulus for laughter is not a joke, but another person.
This is also the basis for adding Laughing Tracks in TV shows--to make you feel like you're enjoying the show with and laughing with others.
The idea that a group makes it easier to laugh and joke backs up what AsheraH has said about not targeting a single person. If more people are part of the joke, it will be easier to laugh at!
Tragedies vs. Mishaps
Peter McGraw, Caleb Warren, Lawrence Williams and Bridget Leonard wrote another article entitled "Too Close for Comfort, or Too Far to Care?" The article proposes that negativity is an essential part of humor--that humor is only made possible by violating some norm or rule. They examine how breaking some rules too far can turn a joke from funny to disturbing and turn laughter to hate.
They specifically focus on "Tragedies" and "Mishaps." From the abstract:
Five studies show that tragedies (which feature severe violations) are more humorous when temporally, socially, hypothetically, or spatially distant, but that mishaps (which feature mild violations) are more humorous when psychologically close.
For April Fools, you'll be playing a joke directly on the person--they'll be directly psychologically close to your violation. As such, you'll want something that does not introduce tragedy. You're looking for something relatively benign.
In addition to violating some norm or rule, one cause for amusement is a mismatch of expectation and reality. This is the basis for most April Fools jokes.
Professor Rod Martin, author of "The Psychology of Humor: An Integrative Approach" mentions this with one caveat:
This brand of humor only works in a playful atmosphere where the incongruity represents no real threats. There could be things that are incongruous, but they aren't funny. Someone walking down the sidewalk and being hit by a car is incongruous, but not funny."
This ties in with "Tragedies vs. Mishaps," but adds the idea that the mood and the environment the person is in plays a large role. As a boss, setting up a meeting with your employee titled "Your standing in the company" as a joke likely wouldn't work well. They'll immediately be on the defensive, worrying about whether or not they'll have a job tomorrow.
All three of the above are tied together. Having multiple friendly people together will both give the joke more chance at success as well as creating a non-threatening environment. Keeping the incongruities from being threatening will also involve creating "Mishaps" instead of "Tragedies."
However, even if you follow all of these rules, your jokes may still flop--or worse, result in anger instead of laughter.
At the end of the day, you need to know your audience. Some people can take a joke better than others--this may depend on their personality or their relationship to you, but this is something you'll need to judge for yourself.
Best of luck and happy April Fools!