I've already explained how I edit my photos most of the time (you can find that post here). Every once in a while that just isn't quite enough. So the other tool I use in GIMP is under the menu choices Color--->Curves.
Now the use of this tool is an ART, not a science, so I cannot tell you exactly what to do. Just get in there and play! Normally, I just grab that little handle near the lower arrow and move that around until I get what I like. Once in a great while, I might actually only adjust one color by changing the choice near the top arrow. For example, if the red is just too strong, I might go in here, choose red, and tone it down a bit. Here's a card before (you sick of this card yet?):
Those clouds are just a little more vivid.
"Queen of the Scene"