One thing that I sometimes notice about some of my renders is that they seem low in contrast, like I've covered the camera with a veil. That's when I hear the music and realize I'm singing the Low Contrast blues.
There are definately times where you want a lower contrast image for artistic reasons, but if you want to push a higher contrast in your render, to give the image more snap, here are a few ideas I've come across. My goal in this is to try and correct a low contrast scene at the source and to avoid using an image editor's Contrast control in postprocess. I'm still a student of these things, so I invite others to jump in and respond -- or correct me as needed.
Seeing it in a histogram
Besides just knowing something looks low in contrast on your own, you can also use an image editing program to see what's going on. Just check the Levels histogram of your rendered image. You can tell if it leans toward lower contrast if the histogram shows a mountainous shape in just one area. That means the majority of your image uses tones in that one area.
A higher contrast image will have a histogram that's more spread out. You can see the difference in the Contrast Example picture below (the lower contrast example is one of my images, the better contrast histogram was from one of Nancy Gormezano's images from the Showcase gallery).
General ideas to help increase the contrast of a render within A:M
1. In the settings for your Lights, set the Darkness to 100%, this will make the darkest part of the shadows black and help fill out the lower/darker tones of the image
2. in your Model's surface properties, avoid using the Ambience Color and Intensity (unless needed for a specific reason). The appearance of shadows on an object will diminish as Ambience Intensity increases.
3. make sure each Light in your scene is there for a reason, and if you have multiple lights or are using a SkyLight, don't throw away the idea of using a well defined Key Light. With one strong Key Light, you can then use and balance all the other lights as Fill Lights, making their intensities low enough so that they don't together add up in intensity to overpower the Key Light.
4. keep the Attenuation setting for your Lights at 100%. In A:M, this setting means the light intensity falls off quadratically with distance from objects which is what you want. Lower values mean the light intensity remains stronger with distance.
This is a start, and hopefully other great ideas will follow this thread.