Depending on the species of snake, cut bits of worms presented by dropping them just outside the hiding place, where the snakes can see them from hiding, can induce the little nippers to feed. Not all garters like worms, so guppies in a dish could be a second try if yours don't go for worm fragments or small worms. I don't worry much about substrate getting on the food, as snakes are pretty good at scraping off dirt as they swallow. If stuck dirt worries you, try putting the snakes in a clean container and offering food there. Garters are usually agressive feeders and will often eat even after being transferred to a container.
I have also had luck with dead fish, even defrosted fish and fish chunks laid out on the hide bark or on a piece of plastic on the floor of the cage. If you have nuisance feeders like the litter of Ribbon Snakes I'm working with now, and you are fortunate enough to find baby toads or a pond full of small tadpoles, you can offer those. My Ribbons accept small fish either live or dead, and love Gray Treefrog tadpoles. I have gotten the mother to accept defrosted mice scented with fish or frog, and I expect the babies will eventually accept scented pinky parts.