I’ve tried using different or separate apps for this but I always come back to my task manager, which happens to be OmniFocus. I just have a single action project that is just named relationships and have tags for each person’s name. I also have a perspective that shows these relationships. On iOS I can just pull down on the homescreen and type in the person’s name or type relationships and either go to that tag or the perspective really quickly.
One thing that I do to remove all the friction is to specify how I am going to follow up in the task title (call, text, email, visit, etc) and then in the notes field I always put a link to whatever method I am going to do. If its an email I put the email address, phone call or text I put the phone number, etc. This way it removes all friction when I am going to do it. I’ll even go one step further and type in the message that I’ll send them. For example, if I want to follow up with a friend on how his marriage is doing I’ll put the phone number and type out a message like “Hey man, I just wanted to check in about our last conversation a week ago about your marriage. I hope things are going better, keep me posted.” Anything I can do to remove friction helps.
Depending on the follow up I will sometimes set a notification. If it is a simple text, especially one that I’ve written out, I will set a notification that I will most likely not be doing anything too important (i.e. at night after the kids are in bed, Sunday afternooon, etc). The notification is less intrusive because I just have to act on something that I’ve already written out.