Don't vote. That's how.
Fortunately, right now, most Sanders supporters indicate that they will vote for Clinton when faced with the Trump alternative. Here's an insightful comment from Life after Bernie appearing in the NY Times (h/t AZBlueMeanie).
While a few die-hard Sanders supporters have vowed that it’s “Bernie or bust,” a Quinnipiac University poll from late May found that three-quarters of Sanders supporters would vote for Mrs. Clinton if it came down to a Donald Trump-Hillary Clinton race in November. Alex Sheehan, 29, a digital media entrepreneur in New York, said he thought that when faced with the prospect of a Trump presidency, most Sanders supporters would end up voting for Mrs. Clinton — and “If not, we deserve the catastrophic failure that follows,” he said.
And here is AZBlueMeanie's summation.
Andrew Sullivan has correctly described Donald Trump as an “extinction-level event” for democracy, America Has Never Been So Ripe for Tyranny. Voting is serious business, and you had better take your vote seriously. Otherwise you may be an enabler of Trump tyranny. You don’t have the right to subject your fellow citizens to that dystopian future out of spite.
Robert Reich tries some gentle persuasion on his blog with a "Dear Bernie" letter.
I don’t know what you’re going to do from here on, and I’m not going to advise you. You’ve earned the right to figure out the next steps for your campaign and the movement you have launched.
But let me tell you this: You’ve already succeeded.
[Snip. Reich lists the accomplishments of Bernie's campaign.]
Regardless of what you decide to do now, you have ignited a movement that will fight onward. We will fight to put more progressives into the House and Senate. We will fight at the state level. We will organize for the 2020 presidential election.
We will not succumb to cynicism. We are in it for the long haul. We will never give up.
Thank you, Bernie.
Bernie has already won the ideas and won a position of influence at the Democratic national convention. Now it is time to win votes in the general election. Now it is time, in my opinion, for him to join with other Democrats and prevent the "extinction event" known as Donald Trump.