Starting in 2010 I wrote one letter per year to a generation of my descendants, ending with the seventh generation. I got the idea during the 2008 financial crisis, when the news was so scary I had trouble imaging the next year, much less the next generation. If you're having trouble taking the long perspective, you my want to write similar letters.
Here are some of the things I had to consider in writing to my descendants. I thoroughly recommend you consider these if you need to break out of your own point in time.
For me, this was the biggest step for success. Without a plan, I would never have gotten around to it. A time and place of solitude is ideal. I chose to write one letter per year on my annual trip to Camp Unistar.
For my first few letters, I wrote letters to my grandchildren to be opened on their 18th birthday. The first is to my first grandchild, since writing to a whole generation seemed too daunting to start. The second is to the rest of my grandchildren.
After that I realized that 18 is a busy birthday, and a few more years of maturity might help them appreciate the letter more. So for generations 3-7 the instructions are to read it on their 20th birthday.
What age you choose has a profound effect on the letter. I was in my late 30s/early 40s, so it didn't feel right giving advice to anyone older than 20. If I were in my 70s or older I might write to my middle-aged descendants. I could have sent greetings to someone my own age, or even instructed them to open the letter after the birth of their first child.
Seven generations worked for me. If my two kids each have two kids, and each generation similarly has two, I'll have 128 seventh-generation descendants. If they each have three, I'll have over 1400. And, of course, I might have none. Similarly, my descendants will have 128 ancestors from my generation, assuming nobody marries a distant cousin. After seven generations, all my unique genetic traits will be thoroughly averaged out. (I'm ignoring adoption and genetic modification, though of course adoptees are invited to read my letters.) I expect people to read my last letter between 2030 and 2050.
The easiest solution is to not deliver it: just use it as a writing exercise. One of the inspirations for this project was a visioning exercise in which people were told to write to someone 20 years in the future. At the time my first child was a baby, so writing 21st birthday greetings made sense. Everyone else had vague, science fiction ideas about the future. Mine had precise names, dates, locations, and situations. I kept it.
If you don't plan to deliver it, it won't be as meaningful and it won't be as sharp. That said, the farther you go into the future, the harder the the logistics. I still haven't printed the letters, though there's still plenty of time. You need to print a lot of copies, since you don't know how many you'll need. I'm still working on figuring out an angle so that it's fun rather than a drudgery. Regardless, I have every intention to get them delivered.
The rest should be straightforward: give them to my kids once they are adults, in waterproof, archival boxes. Preferably decorative boxes that can be heirloom mantlepieces. This makes it less likely that they'll be left in moldy attics or basements. Hopefully after the first generation or two has received their letters, they will look forward to safeguarding the letters.
I also put them online, and made sure the Internet Archive found it. (It's at https://www.leppik.net/david/7gen/ but the text is scrambled until a later date.)
I limited myself to 2-4 pages, so it wouldn't be too boring or daunting. I asked the young folks on Unistar staff what they'd like to hear in a letter written by their grandparents before they were born. This was met with enthusiasm, but few ideas.
Ask yourself what you'd like to read in a letter from your ancestors. Your great grandchildren might like to hear stories about their grandparents. Old family stories might be forgotten and worth retelling.
Don't use fancy language. It doesn't age well. Mark Twain still reads like it was written yesterday. Also, they might not speak English, but they should be able to get it translated.
Consider the breadth of humanity and experiences you may be addressing. You might be the only loving relative in their life. Send them love. Send them hope.
My last letter was written in the summer of 2016. It seems the world changed only a few months later. If I were to write them again, I'd have different things to say. Perhaps in a few more decades I'll write follow-up letters.
Based on email to the First Universalist Cyber-Coffee-Hour listserv on Jan. 29, 2020.