Pruning is about much more than simply cutting leaves and stems off a plant; it's about what happens after you make your cuts. When the tip of a stem is cut, a phenomenon called "apical dominance" occurs. Apical dominance causes branching to occur where the cut was made. Mexican petunias, or Ruellia brittoniana, have no problem flourishing on there own, and some experts actually recommend that you do not prune them. However, pruning Mexican petunias is entirely up to you, and really just depends on how quickly and how much you want your petunias to flourish.
Pull off withered blossoms with your fingers. This will keep the plant healthy and looking good.
Look for the growing point on the stem of the plant and pull off the tip of it with your fingers. This will thicken the plant by causing more flowers to bloom from the same plant.
Cut off dead and dying petunia plants in your flower bed. Cut the plants at the stem just above the ground.
Clean up your clippings. If you leave the clippings in your flower bed, they will attract bugs and animals that can ruin your petunias.