Drew Houston from Dropbox posted a slide about lessons they learned from building, launching and promoting their product. The first lesson was to learn early and learn often. You need to get something out to the users to get feedback. The catch is you need to build something that they want. They also found out that marketing using adwords didn't work out for them because users rarely search for a product like Dropbox. Instead, they rely on word of mouth from their fans to reach new users. The most important was to make the product work as it should.
There are many more insights about how to market a product like Dropbox. For example, it's also important to know the market type for your product and how it fits into your user's life. Check out the slides below.