Also if, say a Senator wishes to run for President (like McCain and Obama in 2008) they should have to resign as a Senator first, and then run for President. Their vacant seats can be filled with appointment or special election.

In this way only PRIVATE CITIZENS can run for President, not Politicians. They can grant no special interest if they lose because they won't be going back to their seat in the government at least until the next election. Also they can not take part in any political policy making (like McCain and Obama did with T.A.R.P. and the bailouts) while they are campaigning for higher office.