I upvote the comments on my posts at about the 5-6 day time frame, not when they are first made. By that time, the commenting is pretty much done.
I review those comment streams - most of the time they have turned into conversations. For each person who commented, I pick their best comment and upvote it. Often, I go visit that person's blog and comment on their recent posts and maybe upvote.
By doing this - the person is reminded of me and often comes back to keep talking or goes to another one of my more recent posts and engages there. This has been working pretty well for me in keeping in touch with people who like what I do.