If you want to get corporations out of government, you need to get government out of corporations.
The payoff for a good regulation is enormous - it can stifle your competitors and grant you a monopoly... It can even create an entire industry.
There is no way on this earth that someone is going to not go after a prize like that. Reduce the regulatory power of the government, and you reduce the incentive of corporations to meddle in politics.
Require a supermajority or even higher for anything that adds a regulation or increases spending. Tyranny of the majority isn't as much a factor when a large percentage of us agree we want to be tyrannized.
Implement the converse proposition - require a superminority to remove a regulation or reduce spending - and I think we'd have an eminently well behaved government.