Disclaimers
tl;dr at the end
My perspective comes from living in part of Burgerland (aka the US) and will be focused on the left "movement" in this country. I'm not going to do the typical American thing of assuming my experiences and perspectives are the only ones that matter, so please let me know in the comments how other countries handle what I'm about to cover.
Also this post has been kicking around my head for a while, so apologies if it ends up long-winded or hard to read. The way I think is all over the place, so I have lots of headers and sections to keep things organized.
Intro: Messaging from the left = Capitalist Realism
Mark Fisher (RIP) wrote a very important book about a phenomenon he called Capitalist Realism.
I highly recommend every leftist reads at least the opening chapter to this book.
In this piece Fisher describes a widespread cultural attitude that Capitalism is inevitable, the only system that can exist in this "end of history". No alternatives are possible, so don't bother trying. And if you do try, you will be made an example of, another reason on a long list of reasons not to try again, so don't even bother. Or worse, your criticisms of Capitalism are used to help keep Capitalism going! Your efforts co-opted and integrated seamlessly into the machine. "Sorry To Bother You", but I doubt "Don't Look Up" got throngs of people who never heard of Socialism to sign up with 
The left, unknowingly(?), contributes to Capitalist Realism
Look at the news feeds, the posts on here, all of the updates on all of your favorite Reddit subs, left Twitter, etc. Overwhelmingly negative stories about how Capitalism is screwing us, how this week a company is caught poisoning food, or how capitalist governments are suppressing speech, or some shitty Conservative politician pushing the latest woman-hating bill. I could go on, but I think you get the point. Capitalism sucks. No one here needs convincing of that.
What I want you to focus on is how this deluge of negative stories affects activists and organizers. More in comments.
That theory doesn't affect me because I can't read
Yes, theory is something to say here and on Lemmygrad or LeftTok or what-have-you. I'm not trying to reach the already dedicated core of leftists, but people who are more left-curious, people who see the contradictions in Capitalism but aren't quite in our camp.
Theory just isn't going to move the masses, mainly because the average American can't read! I'm not exaggerating: check out this Medium article showing 54% of American adults are reading below a sixth-grade reading level! That means the overwhelming majority of the audience we need to reach simply isn't going to understand the Communist Manifesto, much less Pedagogy of The Oppressed or Capital v.1. We need more accessible communication to the average person.
So how do we convey this important information from theory into something that adult with elementary-schools levels of reading comprehension can process? Ideas:
Advertising works with emotions, not facts
I'm kind of a Political strategy nerd. It's not a secret that I am an ex-:lib:, and I see nothing wrong in learning how experts in the field are looking at politics today. I say learn from what the major parties are doing well and adapt those to our needs. I'm currently reading Reinventing Political Advertising by Hal Malchow and am sharing some takeaways from this book.
A good ad should have a goal of eliciting a specific emotion from the audience. Ex: Showing how Cuba, despite decades of embargoes and American pressure, has managed to thrive should cause cognitive dissonance in people who buy into America being the "best" country. Other ads should be aimed at creating feelings of disgust, anger, pride, or inspiration.
We want to always have a specific set of emotions as our goal. We do a good job as a movement of stirring anger and disgust among our adherents. Where we fall short is balancing that with messaging that inspires, gives hope, and helps us build pride in our movement. All anger and no hope leads to burnout and cynicism.
We should build our own audience, not try to win over the Democratic targets
I hope I never get another canvassing turf that puts me in the middle of a upper-middle-class cul-de-sac. That's prime lib territory, and I'm afraid as long as leftists work with the Democratic party's infrastructure, voter files, etc. we will continue knocking on doors that are targets for Democrats, not us. I support electoralism as a way to get messages out more efficiently through a candidate's increased access to the press, but not by running people under the Democratic Party.
One way one could explain why electoralism gets a bad rap... talking to the wrong audience! Bernie tried to mobilize everyone at once. Winning over a population doesn't work that way. We need to adopt a more gradual, long term approach. Changing minds doesn't happen over a single election cycle.
We need to find our own audience of people who are going to be the most receptive to a radical movement away from the status-quo Capitalist parties. To me that means we should target:
Let's do a better job of actively recruiting new leftists
I believe our movement does a terrible job of this, with Trot orgs being one major exception. It sounds icky to say, but I believe we should invest some time and funds into things like Facebook ads, acquisition ads, maybe a billboard or two, a targeted mail series, etc. It works for literally every other political organization in the country, but we don't do it or not enough of it. OK, actually fuck Facebook, don't spend a dime on that company.
There's no ethical consumption under Capitalism, but there are less unethical ways to get ads out. Lots of cities have an "alternative" paper with local concert listings, event guides, etc. Those would be great places to place an ad instead of with the legacy newspapers.
Mailers do not have to be fancy. In fact, Marchow (see a couple comments above) shares data collected by Democratic analysts showing that the most effective mailers were ones coming from a neutral-sounding source, on plain paper, with no fancy graphics and just plain text about the candidates. I believe we could do a similar thing to promote Socialism and see good results.
Misc thoughts