I usually offer food on the tip of my finger for very young babies. That gets them used to you, and for mine they've always been so excited about the food that they forget all about the giant hand in their cage. After they get used to my hand like that, I'll lay my hand in the tank and wait till they happen to crawl over it either by curiosity or chance, but I don't actually pick them up. I don't actually handle them till they get older and sturdier unless I have to.

Also, if you have any adult garter snakes, I think it helps if the new one can see them taking food from you and being handled. They do have a sort of 'herd' mentality, and seeing other garters sometimes makes shy ones more outgoing.