Term limits for senators could work, but it also throws the whole system out of wack if you have term limits that are too small and not enough senators stick around between congresses to get bills authored and passed. 

Also kinda breaks how committees work. It's better imo to just vote out the bad senators and keep the good ones. 

I'm thinking something like decades of service. 24 years for senators and 16 for representatives. This will get them to move up and leave the seat open for another person.