Elliott was an outside cat his whole life.
He showed up as a tiny kitten, from what I've been told he was no more than 2 months old.
In November we decided to make him an indoor cat, by that time he was 18 months old.
This may be a rare occurance, but Elliott screamed for the first 10 or 15 minutes he was in the house and that was it.
He has no desire at all to go back outside, in fact he runs a mile from an open door.
Elliott was and still is a big baby.
He was bullied by the other strays in the neighborhood, if they would get within two feet of him he would run screaming at the top of his lungs to either our back porch or my neighbors back porch where he felt safe.
Maybe it was so easy for us because of the fear Elliott had outside.
I agree with GoldenKitty, time and patience is the key.
I wasn't sure that Elliott would adapt to being inside, but he adapted so fast even my Vet was shocked.
We had three other cats at that time...one has since passed away...they're all females.
We kept Elliott in our spare bathroom with toys, litterbox, bed, food and water.
We left him out to be with the girls a little bit each day...the first day (after he was tested, had his shots and was neutered ) he was out with them for about an hour.
We just kept adding a little time each day to his time with the girls.
About a month after we took him in he was out with the girls all the time and had free run of the house like they do.
I don't know, but maybe that also helped him adapt so quickly.