- Keep a semi-regular communication channel. For me this is easy, it isn't a chore for me to just text people. I know some people find this harder. If I see something I think they would find funny, I send them a link. If I start wondering about something I know they're knowledgeable about, I send them a question. If we have a shared hobby, I talk to them about it. Texting someone even just every other month can be the difference between keeping a friendship alive and letting it rust.
- Make sure to care about them and where they're at. Keep track and a week later ask "how did that interview go?" (for example.) Ask about their lives and sympathize with it, and make an effort to remember. Don't just tell them about you. One really easy way to make a difference is to keep track of people's birthdays, by the way. Just write it down in a text file somewhere if you have to. I know the birthday of everyone in my life - it actually takes borderline zero effort to write it down once and check that file once a month - and I think that makes a difference.
- Meet people where they're comfortable. Some of my friends are happy to jump in discord and just chat. Some would rather phone call every couple months. Some do neither but will respond to texts daily. Don't think like "this method works for my other friends, why are you being difficult?" Figure out what fits them. (And there are some people out there who won't want to do any of these things, and those people can be harder to keep up with. And that's just how it goes. But in my experience those people are very rare. I only know one, personally.)
- Getting along with their chosen significant other is paramount. I've lost two formerly-very-close friends to spouses who I'm not compatible with. You don't have to be good friends with them, but you do have to avoid insulting them or going against their values when you're around them. Eventually you may sometimes have to answer a question for yourself: do I value my friendship with this person enough to accept being around this person I really don't like? And sometimes the answer is no, and again...that's life.
- Over time part of why relationships fall apart is that you're not sharing experiences together anymore. You don't live together in college anymore, for example, so you no longer have that shared experience to bond over. You live a thousand miles apart and don't know any of the same people, so you only care because it's happening to them, not because you're experiencing it too. It can make a huge difference to plan trips together when possible. "Let's go hiking together." "Let's go to Disney together." "Come stay with me for a few days, I'd love to just have a guest. You can work in my spare room and we can hang out at night and make dinners." WHATEVER. ANYTHING. You don't have to go to Disney, you can just go grocery shopping together. That's still a shared moment. Maybe the cash register will be rude and you'll both be taken aback. That's a new shared memory.
And having shared memories is the biggest key.
Last mile problem? Have your personal assistant drive whatever vehicle you want and have it waiting when the train arrives. They can take an Uber back to wherever they need to be next.