It seems to me there has been an increasing number of people coming on looking for help finding their lost cat. Maybe this will help a little.
As soon as you realize your cat is missing:
Go to the Humane Society or shelter in your area with a picture of your cat. Do not phone. You need to go there in person. Kitty may have been turned in or caught. Ask to see the sick room and the feral rooms. A cat who has been trapped/caught may "act" feral. Act quickly. These cats are the first to be euthanized. Usually within two to three days. Keep going back and asking about your cat. S/he may eventually end up there.
Make posters and plaster the area your cat would normally be in and the neighbourhoods around you. It is not necessary to offer an award on the poster. You can make that decision when your cat is returned. Post them in vets' offices, in malls, wherever you can.
Go to your neighbours and ask them to check under their decks, in their sheds, anywhere where a cat may hide out or have been curious about. An open shed door is a greeting card for a curious kitty. Check construction sites in your area. Your cat may have gotten trapped in a shed on site or a closed room which may not be opened for some time.
Put out kitty's favourite blanket, litter box, and food. You can also add an article of their favourite person's clothing recently worn. This may help call kitty home.
A can of sardines, tuna, or salmon may help lure out kitty.
Go out early in the morning (4 - 5 AM), and late at night (11 - 12) with a flashlight and a bag of kitty's favourite treats. Shine the flashlight into bushes, hedges, under places where kitty may be hiding. Call kitty's name repeatedly. It is quieter then and kitty may be more apt to come out of hiding. This has worked often to bring kitty home.
Buy or borrow a humane live trap. You can set it up on your porch, in your back yard, in a place where your cat has been spotted or would usually hang around. Bait it with the tuna, salmon or sardines. If kitty is a catnip hound you can even use catnip.
As much as this may hurt call whoever is responsible in your town/city for picking up road kill. In this area the organization is called Public Works. Ask them if they picked up any cats and the description.
If anyone can think of more to add to this please do. Let's help bring cats back home.
Good luck. Don't give up.