That's excellent advice Mike. I heard about this and implemented it to my sites many years ago. I think I got the tip from the Article Underground forums because back then this was my only resource for learning all about Internet Marketing (IM).
DON'T LOSE YOUR LINK POPULARITY SCORE, AKA PAGE RANK
Before adding this code to the .htaccess file, my site's home pages (the page with the most back links) all could be opened as follows:
And to prove that they were all getting their link scores separately the page rank on the most popular site of the day went like this:
Home page 1 – PR4 Home page 2 – PR2 Home page 3 – PR1
When I did a backlinks check I could see that the total links to the site's home page were distributed among the 3 domain formats.
Nowadays, it's impossible to open any of my sites home pages in any way other than my chosen format which is http://www.mysite.com/, and this was achieved by adding that simple bit of code, that Mike talks about above, to the .htaccess file.
It's literally a 2 minute job and it's comforting to know that neither links nor page value is getting watered down.
What this means is that if my site gets linked by other people around the web, I can rest assure that every single new link is going to point to a single domain format.
So there we have it. By adding this code to the .htaccess file your sites pages are not getting duplicated and devalued, and neither are your backlinks getting lost to the various formats.