How do we stop the wealthy from buying up all of our political candidates?

I have worked for a couple of billionaires over the years as a CTO. I know that I would never ever let them make decisions for our society, because I don't think they have the capability to make decisions that really benefit anyone but themselves. I also want the political process to be influenced by the average citizen to ensure that we have a democracy. In the USA where I live, it seems like the wealthy control our politics. Why does this happen? How can we prevent this? How can we ensure that the politicians are working for the majority of citizens and not just the wealthy?