This short cartoon gives a good history of why corporations have so much power and what it does to our democracy.
We're not crazy about the solution it offers up - unless an amendment campaign sets forth to abolish ALL corporate constitutional rights (they just talk about the 1st Amendment in this video) we won't have made a whole ton of progress.
But this is a good way to start the discussion...
If you'd like more info about why and how ALL constitutional rights for corporations should be addressed, see
"Why Abolish All Corporate Constitutional Rights" and "Not All Responses to Citizens United Are The Same".