This is probably the single biggest difference between you and a professional photographer.

Here is the simplest trick:

TAKE MORE PHOTOS!

Next time you take a photo -- go ahead and take 9 more of the same subject with a little bit of a different composition -- maybe a slightly different exposure. A new white balance maybe? Out of the ten you take you have a pretty good chance of getting one or two that are really good. You can delete the rest. Take advantage of the digital world!

That's it. Make it a rule that you will take ten times more picture and will never take just one picture of any given scene!