That's a good question. I know my comment sounds defeatist. Another commenter suggested requiring trades be announced in advance. Sometimes blind-trusts are used, but there's not much to show they make a difference. Opening up hearings and Congressional communications helps. Honestly it's a tough problem.

I don't think there's any country that has truly solved these issues; at best, they mitigated them. Politicians everywhere are quite a bit richer than your average citizen.