Of course, you want to get your photo the best it can be "in-camera". Try to get rid of any distracting backgrounds, have good light and the hardest thing, is a willing participant! LOL! My son indulged me here (I owe him a new video game now)
Here, I darkened it up some and added a bit of saturation. Finally, I added a vignette. That darkens up the edges and draws more attention to my subject. I LOVE vignettes and put them on almost all of my portrait photos.
iPhoto is great for photo organization, but it is also pretty powerful for punching up your photographs. Play around with it and have fun!