A few months ago, I ran into the Bump team on Castro street in Mountain View. They were grabbing random people walking by their table, testing the user flow of this product with a demo.

Two lessons from that:

1) Gather user feedback all the time, at any stage. Meet customers, send out surveys, interview users, whatever it takes to get the data.

2) Simplify! Your product needs to be as frictionless as possible (I think I repeated this like 7 times when I tried the demo). It's two steps to use the new Bump photo app -- that's easy for a user to understand.