If your current business model is no longer effective, it's time to make some changes. When you're designing or re-designing your business model, keep these questions in mind:
Does the model generate margins superior to the competition?
How easily can your idea be copied?
What can you do to slow down the competition?
What happens if a larger competitor copies your idea?
Are you aiming for non-consumers or to have existing customers of a competitor switch to your brand? A is easier than B.
How will you get the product into consumer's hands, and how easy will it be to get consumers to pay for it? Think about Twitter and other great products that couldn't monetize the offering.
How will changes in buying habits, trends, interest rates, increasing competition, and the general economy affect your model?