I got my baby Gus when he was one year old. He'd been badly abused before I got him and when I got him home, I'd only see him at night. He'd hide and growl and hiss whenever I peeked under the bed for him. I would sit next to him and talk to him and slip treats under the bed.
In about three months, he would come out from under the bed even while I was around although he wouldn't let me get near him.
A few months later, he wouldn't run away from his spot on the floor if I stood up or moved (although he still ran if he thought I was coming toward him).
It took almost a whole year but he's become the sweetest, most affectionate lap cat you could want. He's still afraid of other people, but he's even allowed my foster son (who's been in the house a year) to pet him now and he lets me walk right up to him and pick him up (although he doesn't much like being held - I think he still feels a little vulnerable in that position).
It just takes time and patience. Love4himies is right - you should probably let him make the first move. Anything you do will probably be perceived as threatening. If you choose to talk to him while he's under the bed, don't reach toward him, or even really look for him - just sit next to it and slip treats in.
Storm (beeps and has a pink nose and toes),
Misty (with big blue eyes and tasseled ears),
Anya (only ever called Honey - she's that sweet),
Zander (who goes by Punkin' cause he's almost that bright)
Kasee (couch potato who thinks she's a dog)