For many years now I've been advocating a process that I call "Deep Cleaning". It involves using a good bore solvent, I recommend Hoppe's #9, a new or like new bore brush of the appropriate size, a cleaning rod, some patches and a little time and energy.
You may read the detailed process here http://allaboutshooting.com/articles.php?tPath=39
Literally hundreds and hundreds of readers have reported to me on the differences they've seen before and after deep cleaning their guns. It's the least expensive single thing that you can do to improve the performance of your shotgun. I recommend that anyone attempting to get better patterns try deep cleaning first. It may be the only thing that you need to have a better patterning shotgun.
Please take a look at the "before and after" picture that accompanies this article. It was sent to me many years ago by a reader who wanted me to see the results when he cleaned the bore of his gun. I've also been sent countless pictuers of "gunk" that readers have removed from guns that they thought were clean.
Normally, you'll only need to deep clean your bore one time. After that a routine cleaning will take care of removing the accumulated plastic and unbruned residue from the shells you shoot. If however you want to deep clean your gun more often, go right ahead. It's not at all harmful and will help you keep your bore pristine.