When I have that moment of realizing how long it's been and the guilt rushes in, I try to take a step back from the feeling and ask myself: how important is this relationship to me? Is it worth being uncomfortable for a little bit? And if the answer is yes, I text them immediately before I forget again. Because it's always better to reach out late than never. And most of my friends understand by now anyways, because I have talked about my difficulties regarding this with them before.
But for trying to stay on top of it I'm not really sure myself. I thought about maybe incorporating some reach-out-to-friends time into my routine, like an hour every other monday where I spend some time calling/texting a friend, but I can decide who to reach out to and what method of communication to use in the moment, so I feel less pressured. Haven't tried it yet, though. Sometimes I like to write a letter or postcard to someone I haven't seen in a while. It might be a little weird and oldschool, but it's easier for me to take my time and have some room to talk about what happened during the last few months. Also it is a nice surprise for your friend to find in theur mailbox!