Build an awesome product and figure out the business later.
Keep design in mind from the beginning.
I'm starting to realize that these statements are mutually exclusive--that if you don't understand your potential market and how you might make money one day, you're going to have an awful time trying to bolt on a business to a consistent design.
This doesn't mean you have to implement your business model day one, but your design is betting on a monetization strategy and a market whether you're conscious of it or not.
It starts with designing for a particular audience. You have to keep the user in mind when you build something. Making decisions about who that user is going to be, what they value and what value you create for them directly relates to your business model.
Not too long ago I met a team that was gathering some fantastic information about air travel--everything down to what airports had that best transfers and whether there was better food before or after security. I didn't think I was the target market. I'm a pretty low maintenance flier. I pick flights based on cost and time/date. That's about it.
I think thats the way that the majority of people fly, but I do think there is a small subset of fliers who would pay just about anything to have a convenient flight with a better terminal experience and more legroom My advice was that they should be a premium service--but not just to throw up a paywall, but to look and act like something VIP.
This would dictate an almost Gilt-like feel where it's all about the velvet rope and feeling like an insider. That would be very different than presenting the data like Mint.com, which is meant to be accessible. That reflects Mint's free model. Those are two very different design esthetics.
So who are you going to be to your customers? Should they be able to tell from the moment they download your app or hit your landing page that, at some point, they're going to need to pay for this service? Are they going to be surprised when ads start popping up?
Moreover, what drives your business model? Is it reach? Data? Signups? How do you design your flows in order to maximize the business *potential*, even if it's not a business quite yet?
I wouldn't even hire a designer without first having at least a guess as to what's going to drive your business in the future, even if the details of it are sketchy.