There are a few options
in general DEEP edges work better.
Shaped edges (see TechKnitter
--her blog or on ravelry)--for a detailed description and WHY it works
Go with the roll
instead of garter, you can use 3 rows of stocking knit, 3 rows of reverse stocking knit, (repeat for a total of 12 rows--or repeat again for 18 rows!) this edge will corrigate.. the controlled rolls will make a great edge --
you might want start with 24 (or 36) extra stitches, and decrease at the beginning of each row (and latter pick up stitches on side and work the same edging OUT, (increasing 1 stitch at each edge as you do) and then sew the mitered corners-- a very pretty edge!
a basket weave (or for that matter, any stitch pattern that has a balance of knits and purls (like ribbing, or seed, or double seed, or the controlled roll of the X rows of stocking knit/X rows of reverse stocking knit) works..