2018/11/12: today, each Congressional district elects just one person, in a winner-take-all election where you only need to win by one vote. This means that the losers end up with a Representative who simply doesn't represent them. This means that, in a close election, 49.9% of the voters can be effectively disenfranchised. Even in lopsided victories, where 70% of the voters support the winner, the remaining 30% are stuck with someone who doesn't represent them.
The solution: elect TWO representatives from each Congressional district, and award them each a fractional vote in Congress. Each of the top two vote-getters would have a Congressional vote that is proportional to the number of voters who supported them. Thus if a district elects a Democrat (D) with 55% of the vote, and the losing Republican (R) gets 45%, both of them go to Congress, and D gets 0.55 votes while R gets 0.45 votes.
This will double the size of the House, to 830 members. It will also completely fix partisan gerrymandering.