1) Our intel failed.
All media is biased, but we read the stuff biased towards our own viewpoint and ignore all the rest. We need to keep up to date with the media biased against us, for two reasons: a) despite the bias, it might be reporting on something we wouldn't otherwise know; b) we need to know what the right wing are hearing, so we can counter it.
2) Electoral politics is important, but it's only one tiny part.
People matter more than polls. We need to spread love and peace where there is fear and hatred, and that can't be restricted to election season. In particular, whenever the right's policies hurt ordinary people, as they will, talk about it to those people. Hear their stories; tell them ours.
3) It's not a game.
Many people on both sides talk as though we're having a football match between the red team and the blue team. I was once at a count where candidates from the left were saying that the opposition's policies would bring hunger and homelessness. The opposition party just booed. They need to learn that there's more at stake than honour, or even principles: people are going to lose housing, heath, and food because of this vote.
4) Angry white people won this election.
Take hope in the fact that white people will be a minority soon! In the meantime, how can we dissolve and deflect this anger, this prejudice, and this fear?
5) Right-wing voters aren't fools.
They're misled, they've been duped, but they're not fools. If you talk as though they were, they'll just assume the left is a load of smug bastards, and hate us more. Especially if you talk about voters without a degree as if they were voters without a brain.
6) People need stories, not just facts.
The right has sold them a story about scarcity: money is scarce, housing is scarce, jobs are scarce. It's a lie: the scarcity is artificial. But people can't accept facts that don't fit into their stories. So, tell them a new story, a true one, and give them facts to support it.
7) Every revolution brings a counter-revolution.
We've done fairly well in the last few years, and this is the predictable backlash. As I said, it won't last, though while it lasts it'll bring injury and death to the most vulnerable people. Let's make sure it doesn't last long.
8) The left is more than just "not Trump".
And this is our chance to move the window further left. Everyone can make some difference wherever they find themselves. Everyone should be as strong as they can. That includes you.
9) Nobody's ever said "no" to Trump in his life.
More people voted for Clinton than Trump, which means there's a lot of us to say "no" as loudly as we can. Help out the ACLU, because freedoms aren't free. And the midterms are in 2018, so make sure he starts to hear a lot of "no" from Congress then.
10) Four years from now
...in 2020, there will be elections in both the UK and the US. This is where we win back lost ground. Go for it.
Comments welcome. If you liked this list, share it: thank you! (Edit: this list was written by me, Thomas Thurman, since people were asking)