You don't. They are human too, tempted by greed or afraid of what happens to them when they are done serving.

It goes along with WANTING to be in a political position, if one did not want the money or the power trips then that one is likely also a nutjob of zealous flavor.

The flaw with this logic, is that the ideal politician does not want the job, this could make them incorrigible or careless. It also means almost nothing gets done by the incorrigible politicians (which I have to see as a good trait for any government) and poor quality work is done by the careless ones.

There's Buddy Roemer. He's better than pretty much all other candidates including Ron Paul when it comes to campaign reform. No superpacs. 100 dollar donation limit. It's in every citizens interest(regardless of party affiliation) to make money less influential in politics.

Buddy Roemer on MSNBC's Morning Joe: http://www.youtube.com/watch?v=8oMoAMnFczY

Then there's Lawrence Lessig. He's not a politician but he's also working to limit systemic political corruption.

I'd rather put my efforts into reducing the power of the President and move toward more of a parliamentary democracy in the US, which will increase the number of parties and diminish the power any particular party has, and therefore companies won't have such a lock on legislation.

Considering it's illegal to take campaign contributions from corporations, I don't think you find many serving politicians who are doing so.