When I got my new old dog (saluki/bull/whippet) last year I spent the whole of the first day I had her, and a great deal of time in the following days, just sat talking to her, giving her little treats, going away and comming back with food, giving her a brush down and playing a bit of fetch and what not and I didnt let her off lead for two weeks.
Spend some time with the dog and earn its trust