A technique for parallel lines that I've been playing with involves using Illustrator, and works really well if you want to put in the time. This is a quick version, I can do a longer one with images if folks think it would be useful.

1) cmd-click the layer with your land masses, bringing up a selection. Create a new layer and fill the selection with black. You should now have a silhouette of your land. Fill the remainder with white, then select all and copy.
2) paste into Illustrator and trace image. Your aim is to get as accurate a reproduction as possible. Expand and delete the white bits so you have just the black left. Select all and mark the fill and stroke to none. If you have multiple land masses, you need to create a compound path: while everything is selected, click through to Object/Compound Path/Make (or Cmd+. (which for some reason keeps showing up as the "cool" emoticon. ???)
3) while still selected, open the appearance panel and click "add new stroke." I made a 1pt in dark blue, which just outlines the edge of the land.
4) keep going: select "add new stroke" and this time make it white, which will give you the space between strokes to create the effect. In this case I made it 2 strokes. This layer should go UNDER the previous layer or else it will cover it.
5) black stroke, 2.5 pt; white, 3.5 pt; black, 4 pt; white; 5 pt. and on and on until you give up or are happy. When you're done and you love it, open the "Graphic Styles" window and create new graphic style. This will keep it forever so you don't have to rebuild it all the time.
6) keep the land mass as no fill. Select all and copy. Paste into PS as pixels, make sure it's aligned with your original land mass, and then you can control the opacity and blending options to make it awesome. Multiply will work well. (There are likely other good ways to transfer the image, this may not be the best. I'd like to hear other ideas).

Here is an image of a huge coastline effect and the stroke values that made it. I'd be happy to post the graphic style so others can use it, but I don't know how to attach non-image files.

This is probably confusing. It's late and I'm done for the day. I'll check back and if y'all want more on this tutorial I'll spend some time on it.

coastlineexample.jpgcoastlineappearance.jpg