Dharmesh over at OnStartups just released a book on InBound Marketing. I have not picked it up yet but the posts he does on his OnStartups blog are fantastic. This post does mention his book but also has some good advice on how to build barriers to entry.
Barriers to entry are what you throw up (or naturally occur) to prevent others from encroaching on your market. The best one, which does seem obvious, is “Create Content that Kicks Butt.” Even though it’s obvious, he gives some good examples of the types of content to create and what they experimented with at his startup, HubSpot.