If you are an NAPP member, there is an interesting tutorial on the website about using a gradient layer style to create the appearance of a double stroke.
The technique is to fill the stroke with a gradient, set the style to "Shape Burst" and then bring the gradient end stops together somewhere toward the middle.
This works in PS CS and CS2. I don’t know about earlier versions.
I also need to do this occasionaly and have used this short procedure:
1. Ctl-A, (Select All), Stroke "Inside" with the width you want and a color you do not find in the image (such as bright red),
2. Deselect the area and reselect with the magic wand tool.
3. Fill with white
4. CTL-Shift-I (Inverse selection)
5. Stroke "Inside" with the width you want and color you want.
Image–> canvas size (center checked and increase by desired amount in height and width. Fill with black.
Repeat but fill with white.
Quick & dirty but it works.
A variation of John’s:
CTRL+A > Edit > Stroke > Inside > adjust other settings > ok
