I have just adopted a cat of one year old and what i did I bought a Harness and took him around the garden afew times each day for about a week.
After another week the weather being nice I worked in the garden everyday while he was having a sniff round and I kept an eye on him.
We have had him 2 months now and he is fine.
We have a cat flap but at present he has not find out how to use it. Bless!! He will have to work that one out himself!!
It is a real worry and a few times i did panic.
He has a bell on his collar and so i can hear him most times.
Also I put a Loc8ter on him,
You attach it to his collar and you can locate him up to 400 feet with an remote control.
That does take alot of the stress a way.
You can locate your cat if he gets into somebodys shed or house even.
They cost about £40 for 2 locaters and you can buy them off the internet.
I bought mine from the NEC cat supreme show last year.
Hope this helps