The most effective way to reduce the feral cat population is to implement a Trap Neuter Return plan alongside the low-cost spay/neuter clinics that exist in most communities. Trap Neuter Return (TNR) is the practice of trapping feral cats, vaccinating and sterilizing them, adopting out the friendlier, more socialable ones, then returning the remaining cats back to the colony. Once the cats are released, a volunteer will feed and monitor the colony. The goal is met; the cats are at a lowered risk for disease, have access to adequate food and the population will decrease as they die from natural causes.
As long as the members of the community are on board, spaying/neutering their own cats and keeping unaltered cats from roaming free, the success rate for the TNR program will be high. There isn't much extra effort required, as responsible pet owners will have already spayed/neutered their own pets and encourage others to do the same.
It's impossible to trap every cat in the colony and those that have been caught once can rarely be caught again for upkeep of vaccinations, but the risk for disease is still being lowered.. If our community adopted this program, we have a chance to stabilize the population of feral cats.