Inspect the screens on a regular basis to ensure there are not tears or openings that insects can get through.
Repair the cracks in the decking if they are not supposed to be there. While there are naturally cracks in a slatted deck, a larger slab often develops cracks that are not part of the design. Filling these cracks limit the options for insects getting inside.
Spray a pesticide around the porch to make the environment more hostile to insects. Follow all instructions on the package to ensure safe results and verify before you apply it that the chemicals won't pose a risk to any plants, people or pets in the area.
Hang a bug zapper on the porch. Moths and various insects are attracted to the light and they are killed as soon as they make contact with it. The zapper helps keep your porch bug-free.
Place bug bait near any cracks in the decking that are not repaired. This lures any bugs coming in through those cracks and it then poisons them. Bugs will also take the bait back to their main nest in some circumstances and kill off more of their population as a result.
Burn citronella candles on the porch. This helps keep mosquitoes out of the area and the scent of the candles dissipates through the screens since the porch has a lot of natural airflow.