Robust primaries. Challengers for every multi-term incumbent.
Get rid of the termers.
I'd go a step further - a national Primary day. I lived in Georgia during the last election. By the time Georgia had its primaries, my effective choices were down to McCain, Romney, or Huckabee, none of whom I thought were worth anything. I voted for Fred Thompson, even though he'd already pulled out of the race. My first choice was Duncan Hunter, but he'd been out of contention for so long that he wasn't even on the ballot anymore.