If you are male, make sure the seat is up, unless you are at a urinal. Be sure both both feet are firmly planted on the ground, and maintain proper balance. Check if the organ in question is outside of your pants - this is crucial! Use both hands to aim. Do not laugh. Avoid distraction and maintain concentration on the task at hand. And never, never bounce a tennis ball during urination!

See also How to Use a Urinal.

If you are female, I can offer no advice