Increasing the canvas size is another useful way to create borders. Select the background color you want, then increase the canvas size equally both horizontally and vertically. You can even use this technique to get creative and make 2 or more background colors around your image. For example....you can set your background color to white, increase the canvas size by 10 pixels on both sides, and then repeat with black as the background color and increase it by another 50 or so pixels.
Note... I'm at work now, not in front of my home PC with photoshop. When choosing your color before increasing canvas size, you may need to actually fill the newly created canvas with the color of your choice. I can't remember offhand if PS does it for you when increasing canvas.
*Edit* - I just noticed that "AMcNeice" uses a similar technique in a thread lower down in this section of the forum. I just wanted to point out that I was in no way trying to rip off her idea...I hadn't even seen it yet.