the easiest way is, just add some layers of adjustments, like gradient owerlay, photo filter and all that shit from "layer/layer adjustments" and then just blend them in, forexample you put darkblue-skyblue gradient over it, and where is blending options they're set to normal, just look under which one would look better, for example overlay, soft light, hardlight, darken, multiply, just brwose thro them and experiment.
Atleast that's how i blend stuff in, and looks pretty cool :]
btw i do that with photoshop ;p