By far, the most effective thing you can do to improve your map is to stop the lurking and post a work-in-progress thread here.

Now for some more specific advice. If you're concerned with your coastline looking sufficiently realistic, why not stitch together a few real-world coasts? Get your general shape, then find some actual coastline that roughly fits and use it as a template to guide your coastline building.

Try not to get too discouraged when you see the breathtaking art that others create. It's unreasonable to compare your first piece to someone else's 100th or 1000th. I do the same thing, and it's a huge block to my creativity. Those masterpieces are the destination, not the starting line!