Ziggy hates me take lots of pictures of him, but he always looks cute in his pictures!
I try and get him in the middle of playing, when he's most riled up! Then I hold a ball or toy above the camera and take a quick shot. That's how I got my profile picture. =) You get the absolute best expressions that way!
My recommendation -- if you have a DSLR camera, use a telephoto lens. Often times, the better pictures I get of Bentley were shot with a Canon 70-200mm. I find that the more distance you put between you and your dog, the more likely he/she will be willing to relax and smile.. instead of being tempted to walk-up toward you or become frightened by a camera covering your face. I also keep treats handy, so when I get good shots I reward him with a treat and that shows him that he did something good. And in a situation like that, he'll remember to do something similar next time.