To have Non-poisonous snake, such as the common garter snake is not a bad thing other than to see one slither around your yard or make their way into your home on occasion. Garter snakes are not poisonous. Certainly here in California we have rattlesnakes and they are poisonous and need to be removed quickly. Call your local animal control for removal. The upside to garter snakes in the garden is that they eat pest insects, mosquito larvae, slugs, snails, crickets, and mice. However, if it is your goal to remove them then there are a few steps that can be taken to eliminate their habitat in your backyard without harming them.
First examine what garter snakes like in a yard
+ Low-growing hiding spots near your house (shrubs, plants, etc.)
+ Thickly-planted gardens
+ Mulch (especially straw mulch) in the gardens
+ Tall grass (Keep the lawn neatly cut and clean)
+ Bird feeders (which attract mice & insects, which attract snakes)
+ Ground-level water sources (birdbaths, ponds, puddles, etc.)
+ Compost piles
+ Wood and brush piles (Snakes need cover for protection)
+ Eliminate piles of rocks.
+ Keep firewood stacked on a rack at least 12 inches from the ground
+ Remove junk piles
+ Keeping the yard clean and tidy keeps the rodent population down and consequently reduces the food source for snakes. In addition, be sure to fill in holes around pipes and foundation. It only takes a ¼ inch hole for a snake to crawl into.

A good screen door would be beneficial. Keeping the door open for long periods of time increases the opportunity for a snake to crawl into the house and what a shock to find it on your kitchen floor!