If you're using photoshop, you might be interested in this method I came up with: http://www.cartographersguild.com/sh...n-antique-maps

there's also a lot of other ways to approach it as well, the most popular version involves copying the shape, expanding the selection and filling on a new layer, then adding layer styles while setting the fill to 0%.. repeat that a few times and you get a nice set of rings.