Responding to the OP's comment that the eventual use would be a banner for a web page, I took the same general approach as IamSam, but added a few of my own personal touches:
1) The 1st thing I did was to clean up the paper texture (or whatever it was) as well as the dust spots / blemishes on the large size version of the image.
2) After I moved a copy of the sun and clouds down, the edges of the sun, clouds, lines indicating movement, etc. were all looking a bit soft, so I over-painted them in a very dark blue-black.
3) When the sun gets close to the horizon, it looks larger than when it is high in the sky, so I increased the size of it and the surrounding clouds a bit. I also flattened them very slightly for a bit more realism.
4) The whole image looked like it needed a more well defined black point and contrast, so I used a curves layer to accomplish this.
5) I thought the relative luminosities and saturations of the different colors was a bit soft, so my final step was to use a hue/sat adjustment layer to tweak these to my liking.
Since this is a learning discussion group, not a free photoshop service, I also downsized the final image to 400 pixels across.
HTH,
Tom M