Once when my family and I moved house we found that our new home came equipped with a cat. Upon asking around the neighbourhood we found that the poor thing had been the pet of the former inhabitants. The house had been uninhabited for quite some time, so needless to say the little boy was quite wild by the time we came along.
However, the yard was his home and he wasn't about to leave for anything. So we decided to adopt him. We started by tempting him into the house with treats, he was extremely mistrustful of humans so we could only do this by leaving the treats and retreating to an apropriate distance. Then, once he was used to coming in the house again, we slowly got him to take the treats from our hands. Eventually he got used to us, so much so that he even let us take him to the vet for a check-up.
Tom (as we called him) lived with us for a long time until eventually he succumbed to cancer. He always kept that slightly wild look in his eye, but you couldn't ask for a more loving or companionable little furry friend. I've had a lot of cats since Tom but I've always had a special spot for him in my heart, I even named a kitten after him, Tommy, who had much of Tom's manner (and looked much like him too).
My tip for keeping your adopted darling, just lots of love.