I'm a bit of an outlier here, it looks like. I do use bookmarks, but I also keep track of a lot of information threads with them. So what I rely upon to keep track of posts owed is e-mail notifications. Of course, if you are involved in busy group threads, you would need to choose the notification option that sends an e-mail for each post, which might be more cumbersome. For me, I have an e-mail account I only use for post notifications, I only do one on ones, and I delete all the notifications as I read them for everything other than roleplay posts owed.
One other advantage to this is you can usually read the posts in your e-mail, if you are not on site.