Glenn Harlan Reynolds has a USA Today op-ed explaining his proposal for a tax on the post-government earnings of government officials:
After all, when it comes to your value as an ex-government official, it really is a case of "you didn't build that." Your value to a future employer comes from having held a taxpayer-funded position and from having wielded taxpayer-conferred power. Why shouldn't the taxpayers get a cut?
More significantly, it is a principle of economics that when you tax something, you get less of it. So if we're worried about revolving-door government, we should tax it, so as to get less of it. And since the revolving door generates bad effects for society, taxation would be an appropriate way of discouraging it.