I just got a wild garter. Here's what they told me to do to tame them. Put them in their cage for the first 7 days, do not pick them up. Be sure to clean their cage each time it gets dirty. After 7 days, pick them up 2-3 times per day, for at least 10 minutes each time. The longer you handle them the more tame they'll get. I've been picking mine up for about 2 weeks now and he just stopped struggling in my hands yesterday, so he is making progress.

Remember they have a primitive brain and act only on instinct, there is really no intelligence there.