By dimensions, he's talking about things like language, living near one another, hobbies, interests, world views (political, religious, and others) and sense of humor. You build those shared connections to begin with by spending time together, Dunbar explains. And by nature some of those ties will become more difficult to maintain (or will just be plain lost) when there's more physical distance between you and a friend.

"Close friendships are one's 'shoulders to cry on,'" he says. "And one around the corner is more valuable than one 100 miles away."

That doesn't necessarily mean, however, that long-distance friendships can't work. But you're probably going to need to find some new strategies for staying close, rather than relying on what keeps you close with friends who live nearby. Here are some tips:

1. Figure out what your friend needs from you

People are different. Personalities are different. And friendships are all different. When it comes to maintaining a long-distance friendship (where certain aspects of a relationship will change), you're going to need to figure out the aspects that are the important ones that can't change.

Some friends are going to take it personally if you forget a birthday or anniversary. For some friends, an annual visit may go a lot farther than monthly phone dates. "Part of being a friend is figuring out what that friend really needs from you," April Bleske-Rechek, PhD, Professor of Psychology at University of Wisconsin-Eau Claire, tells NBC BETTER.

2. Set parameters about how you'll stay in touch

Once you figure out what each of you needs, make a plan about how you'll meet those needs, Bleske-Rechek says. Maybe you decide driving time will be your time to catch-up on the phone, and it's OK if one of you needs to hang up before a natural pause in the conversation. If you have a ten-minute drive, you give each other the ten minutes because that works for both of you.

Some friends simply don't have time (or can't make time) to schedule hour-long phone dates because of the combination of work, family and other personal responsibilities, adds Irene Levine, PhD, psychologist and author of "Best Friends Forever: Surviving a Breakup With Your Best Friend". "Both friends need to be sensitive to each other's needs and desires. The long-distance friendship may take another shape than it did [when it wasn't long-distance]."

3. Remind your friends that you think about them

There's a lot to be said for the random, 3 p.m. text on a Wednesday to let a friend know you just drove past the coffee shop you both love and you miss her. It lets your friend know you're thinking about him or her, Bleske-Rechek says. "I wish you were here. I wish I could spend some time with you today."

4. Remind your friends why they're special, and why their friendship is special to you

"Emphasize that person's unique assets and the way that they're not easily replaced for you," Bleske-Rechek says. It's an affirmation of that person's value to you. We need it in romantic relationships, and we need it when it comes to platonic friendships, too, she says.

5. Talk about the future

Evolutionary psychology research tells us that foreshadowing the future (a fancy way of saying "talking about it") is an important part of what motivates us to put time and effort into our relationships. If we expect someone isn't going to be part of our future, why waste your energy maintaining ties?

We can apply that to friends who live far away by talking about how you intend to keep that person in your life, Bleske-Rechek says. Talk about an event in the future you know you will both be attending or look forward to an upcoming milestone together ("I can't believe we'll both be 50 by this time next year!"), she suggests. "It helps show you're committed to that friendship."

6. Pay attention to the details

Caring about the little things that matter to us is part of what makes a friend a friend. And it's a lot easier to know what those little things are if you're close enough to see them for yourself — your BFF can't stand his new haircut or she's really dreading an upcoming work retreat.

Make an extra effort to remember those details when you do talk about them on the phone or virtually, so you can ask your friend about them the next time you converse. Even if it's way after the fact, staying interested in the details shows that you care, Bleske-Rechek says.

7. Share things about yourself

"Intimacy in friendship is about people letting each other into their lives in a deep way," Hojjat says. What's "new" with you is about more than just the photos you posted on Instagram from your vacation last month. Talk about what you're struggling with. Talk about what you're excited about. Talk about the things that are on your mind.

"Self-disclosure is an important aspect of intimacy in friendship," Hojjat says.

8. Set aside time to actually spend time together, too

Making time to go away together for a few days or spending a few days living with a friend who has moved away is a really good way to boost closeness again, Bleske-Rechek says.

If you visit a friend in their new environment you get to see they day-to-day routine. You get an intimate snapshot of that crazy tennis game or what dinnertime at her house looks like, which makes subsequent phone check-ins and texts more meaningful. Both of you making time to get away together is valuable, too, Bleske-Rechek adds. You both get to leave other commitments behind and just focus on the friendship and things you love to do (exploring a new city, wine tasting, attending a yoga retreat, or what intrigues you).

9. Be there when it matters

Distance makes it more costly to maintain a friendship, both in terms of time and money, but part of what will allow you to maintain that bond is knowing when it matters to show up (in person or virtually), and showing up, Hojjat says. "If it is important to your friend, it should be important to you as well."

MORE FROM BETTER

  • How to make friends as an adult
  • The secret to stronger friendship: Try this to build better bonds
  • How to spot (and deal with) an energy vampire
  • Why we're so obsessed with 'Friends', the comfort food of TV

Want more tips like these? NBC News BETTER is obsessed with finding easier, healthier and smarter ways to live. Sign up for our newsletter and follow us on Facebook, Twitter and Instagram.