The sky:
As a friend here at work put it when I was talking about the amazing light we were having, and the fact that I didn't have my camera with me:
"Just enjoy it, by the time you're ready with the camera, the moment's gone" (loosly translated)
As for composition:
I've become alot more aware in regards to where I put my subjects in the viewfinder, but that doesn't help much "in the spur of the moment" when you're just desperatly trying to get something before it is gone.
What I do in those situations, is just to crop it to my tastes.
On planned shots, I usually plan my compositions pretty carefully, and in some situations I already have a clear picture of what I want, and how to get it. (this for instance

which I had to go back and shoot a second day)
In some cases I've even drawn sketches of the subjects before I went out.
I took alot of pictures of my sister, her fiancee and their kid, and I had quite a few pre-determined shots written down, and just had them pose accordingly.
Sooo, what I'm trying to say is this:
I think about the composition most of the time, but I don't always have the opportunity to get it "right".
That beeing said though, I've taken 3k+ pics so far this year, and alot more last year (when I started) and I was not very aware of how I composed my pics untill pretty recently (3-4 months ago)
The most important part is going out, taking pictures, and learning what you think looks good. That is also important to remember when you come here, showing of your photos. While we have strong opions on how a pic should look, that isn' absolute truth, it's just how WE like it.
(although, you'll quickly learn, as did I that once you start adhering to the compisition "rules" you'll get more exiting photos. I see pics now and I can spot a "tourist" pic in instants

)
-----signature-----
I has a flavor!
CC always welcome.