My filly was born on the welsh hills and had no contact with humans before she was shut in a stable without her mum to wean her, and then herded in a trailer to come to me.
I turned her out in the field and wasn't entirely sure she'd let me near her again! First of all, as shmurmer4 said, as long as they haven't actually had any bad human experience, they'll be curious, so with my girl I just spent time in the field - shovelling manure, talking to the sheep next door, talking to the other horses, talking to her from a distance - moving slowly and thinking low energy the whole time, just so she got to know that I wasn't going to hurt her.
I then used advance and retreat - approach slowly, not directly towards her but in semi-circles, getting near her almost accidentally. Eyes down, shoulders slouched, not looking at her at all. If she moved away, I'd bring my shoulders up, look at her, walk with her, keeping the same distance between us. As soon as she stopped, I'd drop my shoulders, look away, take a step away. Reward her for standing still by taking the pressure off her. I'd then start the semi-circle approach again. Little by little I got closer until I could stand right next to her.
When it came to touching, I did the same thing - when I reached out a hand, if she moved away I moved with her, and when she stopped I stepped away.
I've had her two months now, and from a scared little thing I couldn't get anywhere near, she comes running to me in the field, I can put a headcollar on and off, lead her, groom her all over, pick her feet up, and take her for walks.