In the past I used to use the classic "Contrast" tool, but there are a couple ways which give you much more control, and are less destructive to your photo.
1. Colors > Levels
The histogram you see represents your contrast levels. The black slider on the left represents black, the middle, gray, and the white slider on the right, white.
The general rule of thumb is to slide the black & white sliders to the edge of where the histogram begins to climb on their respective sides. This works best on pictures that include a full range of tones (ie. a photo with some white clouds & dark trees & everything in between).
Photographers, this a tool you should learn how to use. For a great article visit this post. He uses photoshop, but the tool is nearly identical to GIMP's.
2. Colors > Curves
Curves is probably the tool I use the most in GIMP. I make use of it in nearly each photo I retouch. Before making any edits in GIMP, I usually duplicate my background later (Ensure your layers dialogue is showing, right click on the "Background" layer, and select "duplicate".) This allows you to always go back and change something if you don't like it.
The left of the histogram represents your darks/shadows, and the right your lights/highlights. Drag the left end down to increase your shadows, drag the right end up to increase your highlights. Lets say you only want to increase highlights, and want to keep your shadow detail. Select the curve somewhere in the shadows, and place a point there. Then select the highlights and pull it up. This boost only your highlights and will not mess with the shadows. You can place multiple "lock points" on the curve which gives you good flexibility.
If you select the Channel drop down menu in the curves dialogue, you can also change the level and contrast of each of your colors (Red, Green, Blue), which allows you to warm & cool your photos or boost certain colors and not others.
For a more advanced look at curves visit Cambridge in Colour.
If you have any other ideas or comments please post! Always love to learn new tricks.