B.O.’s site has a webform for this purpose:
MyPolicy: What Do You Think?
The best, most comprehensive plan for change in our country will include your ideas and your feedback. America needs a president with a mandate from the people, and everyone deserves a voice in shaping our next president’s agenda.
Take a moment to share your ideas. Over the coming months the best ideas will be featured and incorporated into the campaign’s policy proposals. Be as broad or specific as you want.