1) Introduce more volatity. For example, make a random chance your ambition will not be reached even if you do everything right: You run for X position, and either there's (a) a random chance that 2nd,3rd, or 4th place will win and (b) a random chance that your win will be swapped with another random position (say, winner of public advocate switches for winner of state assembly seat, etc). This would scare away big money from campaign politics.
2)Introduce Sacrifice. Cap lifetime earnings to scare away career politicians, and actually bring people who intend to sacrifice. If you win an election, you sign an agreement with the IRS that any lifetime earnings in excess of your avg earnings before being elected, will be taxed at 99% tax rate.
Watch those "speaking fees" dissappear overnight!
3) Introduce more turnover. Max 8 years as a publicly elected or appointed figure, and perhaps 16 on public payroll for anyone making X times of poverty income level. No one should be able to life in luxury, off the govt, forever.
Always welcome in new blood.