With proper pruning, it is possible to force your petunias to bloom for most of the season. If left to its own devices, a petunia will bloom once and then redirect its energy to producing seed. But you can trick the plant into abandoning its seed production and direct its energy to producing more flowers. The process is called "deadheading" and is as simple as removing the petunias' spent blossoms at the right time.
Observe your petunia plant during its first flush of blossoms. As soon as a blossom starts to turn brown, wilt or look "faded" it is time to deadhead it. There is no need to wait for all of the plant's flowers to fade.
Use a clean, sharpened pair of bypass pruning shears to prune the spent blossom back to where it connects with the main stem. The best time to deadhead is in the evening as the sun is waning.
Check on your petunias daily during their flowering season and deadhead a blossom whenever you see it fading. The more quickly you catch fading flowers the more of the plant's energy you can redirect to flowering.