Message/call them!

Sounds very obvious but whenever something reminds you of them (a location, memory or even just seeing them on social media popping up) just send them a message right away, even if it's just something small and say "I had to think of you".

One of my closest friends moved abroad and isn't on any social media, so I don't get those little reminders of him popping up on my feed. That's why I actually have a bi-weekly reminder in my calendar to give him a call. It sounds a bit silly to schedule friendship, but it really helps us stay in touch.