Spice up that contrast!

So you're trying to make some good looking photos, but really all of them look bland and boring? Sure, you took into consideration several factors to make your scene look better, but still... the colors seem washed. It's ok- it happens with most entry-level photography tools. They're not designed for professionals, but for enthusiasts. I'm going to make a short and comprehensible tutorial about rapidly enhancing the look of your photo using Gimp. The current stable version as I'm writing this is Gimp 2.6.
Wait, I have to drink a glass of water first. Making a tutorial ain't so easy! ...phew, now that I calmed my thirst, let's start this! Here's a close-up photograph of some rocks I took recently:

It surely looks quite bland. What can we do? Well, the first step is to load it into Gimp. Select the background layer and then click Layer>Duplicate Layer(Shift+Ctrl+D). 
Let's make this duplicated layer black&white by going to Colors>Desaturate,  choose Luminosity and click OK. Set this layer mode to Overlay and reduce its opacity to about 50% or around some satisfactory value(we want this effect to be subtle, not to scratch our eyes).
We should also increase a bit the sharpness of our image. To do this, select the background layer,  then go to Filters>Enhance>Unsharp Mask. A Radius=5, Amount=0.5 and Threshold=0 seems to work fine on this particular image. You can fine-tune these parameters, but remember: you always want the effect to be subtle, else the you're effectively damaging the image.

The image is now looking like this:

Basically, we can already save it as it is, it's looking much better than the original. You may be tempted to say it's not, but switching back to the original you'll have a sensation of looking at it through fog...

Let's continue making this photo more interesting!
When I took the shot, I forgot the EV(exposure value) at -0.7, so it's a bit underexposed. Let's profit from this by emphasizing a bit the whites, particularly the bigger white rock. Duplicate the background layer again and move it up(so it's at the top). Go to Filters>Blur>GaussianBlur. Type in a value of 100 and click OK. Set this layer mode to Dodge, with an opacity of about 50-55. To eliminate the possible "too saturated" blurry edges(it's happening on the white rock in this image for example), go to Colors>Desaturate and choose Luminosity to make this layer b&w too. So now we have this:

Now it all depends on your imagination. Would you like to alter the colors or make them more intense? Select the background layer and then go to Colors>Curves and start messing around until you are satisfied. I came up with this: 

(I usually enjoy warmer pictures. I rarely opt for 'cold' colors)

Perhaps you enjoy monochrome images? With the background layer selected, go to Colors>Colorize. Again, fine tune the parameters remembering to make the effect have a subtle impact on the final image. Particularly, reduce the saturation a lot.

 (this looks green and... cold... *sneezes)

Let's try something different this time. Desaturate the background layer( I hope you know how to do it now). Select the Blend Tool(or gradient tool, by pressing L). Set its mode to Overlay and then choose a gradient from the available ones(or you can create one of your own, but the list is quite enough for our purposes). You have to experiment with the Opacity on this one, as different gradients will look better with different values.
For example, I chose the Golden gradient, with an opacity of 70, a Linear shape and dragged from bottom to top:

(now I don't want to hear somebody saying this has the color of puke, because I think it is elegant! Allright? :) )



Or you can go totally wild and do something like this:

(Notice how we completely ignored being subtle here :)... uhm, is that a double rainbow? :O)

Well, what can I say? Editing is a lot of fun! So have lots of it! ;)