We constantly try to get interns willing to work for free at our start up. We get a lot of qualified interns happy to work because we give them interesting work. But if you can leverage your skills and add real value to the start up, ask for compensation. Negotiate a payment plan based on accomplishments. If you meet A requirements, in B time frame, you receive C in compensation. Willing to do work for free up front and take payment on the back end shows that you're confident in your ability to execute and make good on promises.