Enforce strict audits of Congress members income with attention to "gifts" which may have influenced any governmental decisions. Audits should be handled by a few non government sources preferably chosen by the people.
There are rules for what gifts are allowed and it is to the understanding of many those rules simply do not apply.
It would additionally be important to make sure "legal" gifts of above those marks have gone toward the intended purposes (Such as a campaign or fund raising donation goes specifically to the campaign, or fund specified).