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Hi there!! If you don't have a way to rank them, consider developing one. It could breed competition in the ranks especially if this is something you would want to do year over year. Consider the motivation that these bloggers would have for using this particular badge. If it becomes a badge of honor or recognition that could become incredibly enticing those those individuals motivated by such a thing. Other people will be motivated by a feeling of exclusivity instead. Using that exclusivity as a lever in the program can help to engaging that type of blogger. For almost all of the blogging population I would say public recognition is a motivator. Depending on the size of the group you may want to consider making it a public deal...announcement with a list sort of thing to stroke their ego. Credibility is key as well. Ensuring the program is one that actually extends and increases their credibility will make them far more likely to participate. The O'Reilly book - Building Web Reputation Systems by Farmer & Glass will go a long way to helping clarify this in much greater detail. I'd highly recommend that! For a good example check out the Power150 blogger list. Perhaps the best example there is. I also know of many conference organizers who utilize badges in their conference marketing with their speakers. SMX and BlogHer come to mind initially.