Postcards. The best way to keep in touch with interesting people is to send them a postcard when you're traveling.
When you contact them with a request, they'll think about the nice postcard they received from you two months ago.
Because people don't send many postcards, the recipient will enjoy getting something in the mail that isn't the bill. They'll pay attention to your message, and they might hang up the card, and see it often.
The best places to buy postcards is from grocery stores, or thrift stores, where they can be purchased for 10-60 cents each. Postcard stamps are cheaper than regular stamps.
Your message should be short, and congenial. It doesn't need to have an ask, but if it does, it should be very easy. Here are a couple examples:
Just to say hi:
> Hey {fname}, I'm in {place}, and {local_delicacy} is very tasty. Your recent post on {medium} about {topic} was very thoughtful because {legitimate reason, 2 sentences}. See you soon, {your_name}
Make an ask:
> こんにちは {fname}!, That's how to say "Howdy" in Japanese. You would love how many vending machines are everywhere! After I get back in town next month, and I'd love to talk to you about the vending machines you've built. Many of your ideas would be a hit in the Japan market. Talk to you soon! {your_name}
Quick update:
> Howdy {fname}, Everyone here in Montana is so gosh dang friendly! I'm taking some R&R after shutting down my last startup, and I love Big Sky Country. When I get back to SF, I'm going to handle operations at my roommates startup, Tigger: it's like Tinder, but for house cats. Talk to you soon! {your_name}