This quick trick took me a while to figure out, so I thought I'd share it with you.
I've started by giving my photo a simple shadow.
I used to try and add a quick border by using the "stroke" layer style. I didn't like this look because a) it covered up my shadow and b) it gave the photo a rounded edge.
Then, I realized you could stroke on the inside.
Now the hard edge is preserved, and you can see my drop shadow. You can easily alter this look by using a different color for the stroke and changing it's size.