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I am posting this as a separate item because I didn't want to take attention away from the twitvid discussions, and I feel this particular topic deserves its own thread.
I know the obvious, generalized answers: patents, copyrights, trademarks, creating a strong brand, creating a following, blog, PR, SEO, making the product complicated, trade secrets, etc..
What I'd like to discuss instead, are unique ideas for barriers to entry. Also, real-life case scenarios and stories from YC founders and HN users, who have successfully spun-out their start-ups with competition high on their tails, about what they've done to leverage and to keep their first-mover advantage.