Well, let's try to see if we can come up with better ideas. They're not in any particular order.
1. Voting rights. — In a democracy, it doesn't matter how many voters you convince if they're not allowed to go to the polls. Expanding the franchise (felon enfranchisement, enfranchising territories and DC, lowering the voting age, getting rid of voter ID) would have a huge effect on who could vote and who would win elections. The left and the Democratic party have treated this as only a secondary issue, focusing instead on campaign finance reform—a much less important issue, imo. Moving voting rights up on the democrat's agenda could have a huge effect on left success.
2. Working with your side.—Convincing people who hate you is mostly impossible. Working to change priorities of people who generally agree with you is much more successful. Similarly, contributing time, money, and other help to organizations and activists who are already working can create change. It's often a slow, difficult process—ACA took decades, and is far from perfect. But ACA was *not passed* by convincing Republicans. It was passed by solidifying Democratic support.
3. Winning. —DeBoer very publicly said he wouldn't vote for Sanders or Clinton. He should vote for whoever he likes, but the fact remains that when lefter candidates win, left policy benefits. This is the case even when the lefter candidate is not that far left.
Trump lost the popular vote, and is unpopular within his own party. Yet his ascendance has opened the way for open racism and xenophobia in public life in ways that we haven't seen in years. Winning makes your ideology seem good; winning validates your programs; winning makes partisans rally round. Hillary Clinton winning would have done more for the left than a billion hours spent chatting with centrists or right wingers in an effort to persuade them. That's the case even though Clinton is not perfect at all, and would have enacted many policies I personally don't like.
4. Intimidating your enemies. Letting people know that they will lose their jobs if they are Nazis tends to isolate Nazis and keep them out of public life. Showing up in force at a townhall and yelling down your stupid rep who voted to gut healthcare is a good way to make reps worried about voting to gut healthcare. Forcing a racist editor to step down at a national publication tends to marginalize racists. Politics isn't always a nice place; the founding fathers didn't intend it to be. Demonstrating your tolerance all the time may in fact make your enemies think you're weak. Signalling that certain stances are not acceptable, and will be met with shunning and electoral consequences is a lot more effective than attacking your own side in the hopes that a public which is barely paying attention will congratulate you for your honesty and then vote for the people you've been kicking.
None of this is especially revelatory, I don't think. Activists and folks who work for change are well aware that organizing involves rallying friends more than persuading foes. But writers like me can easily be dazzled by the glamour of their own typing fingers, and think that words will transform the evil-doers once and for all. That's not generally the way it works, though.