Enact term limits on Senators.

With term limits in place, Congress will be more responsible toward their constituents because they will soon be constituents themselves. They will have to live under the laws they have created while in office.
Members of Congress will have less time in office to develop financially beneficial commitments to lobbyists and other special interest groups, thereby undermining the threat of lobbyists being a primary influence on legislation.
Since the time of the Founding Fathers, a general consensus states that people, when given power, will eventually be corrupted by it. If Congress has term limits in place, their power will also be limited. Candidates will be more likely to run for the purpose of serving the people, and they would have to leave office before corruption dominates them.