I used to have the same problem, but as long as I keep challenging myself with new skills, goals, etc., in my hobbies, I can stay interested. No matter how much you know about something, there is always something more, something new to learn. Try getting together with friends who share the same hobby to expand your skill set, or take a class or seminar that pertains to your chosen hobby.
In order to stay interested you should really like that hobby. If you don't like you will probably stop doing it. The hobby should be something that is fun and relates to your interests. Hope that helped :)