Summarize:

If you notice shoots that look limp and are not growing flowers, trim them down immediately. Use pruning shears to cut the stems at their halfway point. This should leave at least 2 inches (5.1 cm) of stem from the base of the plant.  Do not remove any remaining leaves on the stem. During your weekly check, you can remove more than half of a limp stem if it looks unhealthy. Use pruning shears to cut away as much of the damaged stem as possible. Make sure that at least 2 inches (5.1 cm) of it remains above the surface of the soil. You can cut down your plants regularly, regardless of whether stems are damaged or not, to promote greater growth. Each trimmed stem will branch out and give 3 or 4 new blooms within 2-3 days. Remove the bulk of the stems, leaving at least 2 inches (5.1 cm) to regrow. This is an effective way to remove deadheads if you have too many plants to pinch off individual deadheads one at a time. After pruning your petunia plants, water the soil just enough so it is damp. Spray a 10-10-10 solution or all-purpose fertilizer into the soil around the plants. Applying 1-2 sprays around each plant should be sufficient to fertilize the soil and encourage regrowth within a week.  A 10-10-10 solution contains 3 main nutrients- nitrogen, phosphorus, and potassium. Do not apply too much fertilizer, as it can be harmful to your plants in large doses. Use fertilizer no more than once a week.
Cut sinking stems to half of their length to encourage regrowth. Trim limp stems down to 2 inches (5.1 cm) from the base if they are damaged. Cut down a third of your plants every other week, if you have a large crop. Add an all-purpose fertilizer to moist soil to encourage regrowth.