This article from Red State web site covers the topic well enough. The quote is from the comment section below the article:
"To a person in the 18th century,"regulate" meant "to bring order, method, or uniformity to" - see definition 2 in Webster's: http://www.merriam-webster.com
.... It's the same meaning used in phrases like "regular infantry", and has everything to do with training, discipline, and practice. In fact, if our founding fathers were writing the 2nd amendment today, they might very well have written "well-trained". It's the difference between having a mob running around barely knowing which end of a gun is the dangerous one, and a group knowing how to aim and shoot accurately, care for their weapons, and coordinate together with others. Every time you see the word "regulated" in the writings of our founding fathers about guns, try substituting the word "trained", and I believe a lot of their writings make a whole lot more sense. For example, the connection between a "well-regulated" militia and the security of a free state makes a lot more sense if you understand "well-regulated" to mean "well-trained"."