…assuming Bernie comes in 2nd place.
Quick answer: vote every two years.
Longer answer: this question is impossibly broad. I’ll divide it into 2.
For social democrats, members of the US left fringe, and others desiring a country more in line with the rest of the advanced world
You already vote every 2 years. You want the US to attain strong economic growth rates and escape from Republican stagnation. Sweden for example outperformed the US in terms of real GDP from 2006-2014. (Sure, the US beat Sweden during the latter 2009-2014 era, but what do you expect? It was a Democratic Presidency.) http://data.worldbank.org/indicator/NY.GDP.MKTP.KD.ZG?page=1
The key Republican challenge is to persuade the middle class to vote against its economic interests. The key progressive challenge is to flip purple states and purple districts wherever they are. Sanders widened the Overton Window this year and last year. So how do we build on that? I’m especially interested in the views of lefties residing in red and purple districts.
For other supporters of Sanders
The political system is rigged. Thanks to Citizens United and a pliable Supreme Court, pampered billionaires call the shots. Sure, sometimes they can be careless: they thought Jeb Bush would be a great candidate after all, even though he had no plan for distancing himself from the Iraq War. So much for vetting or thinking things through. This was a fail on both substantive and political grounds. Set aside Jeb’s flaccid political skills: his campaign strategy was half-baked. It’s almost if these funders suspend their business judgment and are simply enthralled when they brush shoulders with the politically well connected. I don’t know.
At any rate there are 2 goals. Replace Scalia, Thomas, Kennedy, Alito and Roberts with Supreme Court justices with more common sense: those clowns equated money with speech. Never mind that we basically have little idea of what the Koch Brothers et al actually think (why did they give Jeb $100 million?), except via their bizarre and misleading attack ads.
The second goal would be to take down the biggest opponents to campaign finance reform. Mitch McConnell was re-elected in 2014: he likes the status quo because it levels the playing field for those like himself who are bad on the stumps and lack personal charisma. Somehow we have to persuade his supporters in the polling booth that he and his Republican cohorts are completely on the take and really really like it that way. You can draw a direct line between their hands-off attitude towards Wall Street and their bulging campaign coffers.
So vote out the Republicans at the national level, even when a President isn’t running.
Frankly, I’d be happy if the Republicans turned back to the pragmatic and rationalistic interest group brokerage approach of Nixon/Ford/Eisenhower. But the only path to that destination involves the GOP’s complete electoral collapse, some reflection, a spell in the wilderness, and a reckoning. Nuke 'em from orbit.