put everyone's birthday in your calendar, and wish them a happy birthday (at least a text) every year

like their stuff on social media

friendships ebb and flow, as people get married, have kids, get divorced, kids become independent, move away, move back. don't give up on a friend even if they disappear for a few years when they first have kids

try to arrange at least an annual in-person meet up, if in the same city

try to involve them in your interests, and try to take up their interests, or at least be curious

fantasy football is helpful. golf as you get older. baseball games. meet for lunch if you're in their part of town. host parties for events like super bowl, the oscars, stuff like that