1. DO put some effort in. This is the one day of the year where you can playfully take some aggression out on your friends and coworkers and really let your pettiness shine. So put some effort in and don't half-ass it.
2. DON'T give it up too soon. If your "target" believes whatever insane story you're telling them, don't yell "April Fools!" right away. Make them sweat it out, maybe let them ask you a few questions. It'll make the reveal so much sweeter.
3. DO play nice. You're not looking to ruin anyone's career, relationship, property, or life. This is supposed to be fun . . . it's definitely NOT worth losing a friendship over.
4. DON'T tempt fate. Call it superstition, but pretending that someone died or that you're pregnant should always be off the table.