Many garters I've caught try to bite, go bike riding alongside a creek area and flip rocks/logs looking for them is what I do. If you see one on the path spring for it and try to get it into a striking position/mood if you can. They won't run at this point (from what I've seen) and you can gather your thoughts and move about until your comfortable to actually catch the snake, use a stick to proke at it and it should start to slither away, then gently pick it up. Never had problems with this method, sneaking up is hard so I wouldn't bother.