Sorry for checklist, bit tired so just going to fire off what I can remember!
*Make posters and leaflets to give to people. Include a colour photograph. Don't just pop through letter boxes, ring doorbells and talk to people. They tend to remember better that way.
*Leaflet far and wide. They can travel much further than you would think, and even further if they get distracted chasing a female/ hunting then lose their bearings.
*Inform local vets and rescue centres, and distribute copies of your posters there.
*Inform microchip company
*Don't ask other people to check their garages/ sheds, ask if they mind you having a look for yourself. Most other people won't get on their hands and knees to search for your cat.
*Check in bushes and under decking. A scared cat might not come to you, so search thoroughly
*Make regular patrols, even covering places you have been before. One of mine was missing for a few days, and we had walked past her countless times calling her name. She didn't decide to make her prescence known and meow back until a few days in.
Keeping my fingers crossed he turns up very soon. It's so terrible when they do this.