There's a number of things that work, such as making a trap out of a bucket tall enough that the garter can't get out of, make some sort of ramp, chop the nose off a prey item and drag it up the ramp and toss the prey item into the bucket.
other things you can do is flour/tape across door ways (DO NOT USE DUCT TAPE!!!), put a warm device such as a cd player, dvd player or a vcr (DO NOT USE A TOASTER!!!) on the floor and plug it in, the snake should curl up on it. Check your boxes, shoes, slippers, under couches, etc. Good luck, and remember, nothing is escape proof.