[greyhawk]

Around here in Massachusetts, snakes start coming out sometime in April. I think May is the best month, at least for the place I go to. But i'd start looking once temps have been in the 60's for a few days, or any 70 degree day. I don't know exactly what the climate is like there, and it varies from year to year, but I'd say early to mid April would be a good start. But I mean, if your getting a warm spell at the end of March, they might come out early.

Personally, I would look for features on your property that stand out, like clearings, water, trash piles, logs, rock piles, etc. Solid forest is usually not very good because the snakes have an equal chance of being at any given spot. If you have some water edges, or a pile of rocks then I'd look closer to them because they will probably be more concentrated there.
And think about what a garter snake eats...probably amphibians, and amphibians will probably be near water.

Another thing you could do is if there are logs, rocks, or similar things, you can strategically place them near where you think the snakes would hang out, then in a few days, they might be underneath it. Once at a vernal pool I catch snakes at I saw a black trash bag near the woods. It was already there so its not like I was littering, but I put it at a certain spot, and a few days later a milk snake was under it and a garter was inside of it, already in the bag. But ya, rocks + water + frogs = garter snakes.