Don't scoff at the "Auto" button in Photoshop's Levels dialog. Although it might seem designed for newbies who can't work a slider, it can come in handy no matter what your level (no pun intended).
Pick a photo that could use some punching up. (I shoot with a Canon 20D, which tends to undersaturate and undersharpen, so for me this is essentially any photo. :-) Create a new Levels Adjustment layer. Click "Auto", "OK", and just gaze for a minute at the result. You might see the color balance shifted way too much, or an excess of contrast or saturation.
But even if the end result is garish, Photoshop is at least showing you what direction to take. Back in the Layers dialog, select this new layer and gradually lower the opacity slider. If the Auto Levels adjustment made the color balance way too green, probably it needed to be just a little more green; if it supersaturated the colors, probably they could do with a little more saturation. Don't feel embarrassed to lower the opacity to 30%, heck even 10%, until visually it looks improved from the previous version.
With some more practice eyeballing your photos with such a layer visible and hidden, you'll develop a feel for when to apply some extra contrast, saturation, or shift the color balance. But until then, feel free to achieve the same effect via a low-opacity "Auto Levels" layer.