saltedcaramel so sorry he's not back yet
I had hoped you wouldn't need any tips but this is what I did:
The litter being outside is good, also the contents of the vacuum cleaner bag/drum, and hang out his blanket or whatever he sleeps on - his sense of smell may lead him home
If you have a regular milkman & postman ask them if they can keep an eye out for him as they usually know the local gossip & can pass the word around
Register him as lost with the microchip company and let all the local vets know and any animal shelters as well
There is a national website for lost pets, can't remenber the name at the mo but I did register my cat on there (as advised by local RSPCA) - it didn't come up with anything useful for me though
Print off a load of flyers and put them through as many doors as you can - include a request to check sheds/garages as sometimes people don't think to look. Notices on lamp posts don't always get the same level of attention.
Facebook local group is a good idea, and a Twitter-using neighbour also kindly tweeted about my cat being missing
Try going out again with food at night when it's quieter, you may have a better chance of hearing a miaow
The one I found hardest - ring your local council (probably environmental health) as I think most councils keep records of any callouts for animals hit by cars, and I know my council scans for a microchip, or tries their best to find a collar, and they would contact the owner if at all possible. I hated making this call, but at least knowing that there were no reports of dead cats gave me hope that he was still alive & well
It is still early days, good luck, there were 2 really lovely stories on here recently about returned cats & at least 1 of them had been missing for 6 weeks. My cat just turned up again in the middle of the 3rd night and we still have no idea where he had been.