With my Pdoc, I write down everything that's changed whenever I notice it, as well as when I take as-needed anxiety medication, because we always discuss that last thing because I'm afraid I'm taking too many.
Then just before the session I review everything I've written down, cross out everything that isn't relevant anymore or is just plain unimportant, add anything I want to talk about, and bring the list with me.

With my T, I sometimes send her e-mails. Sometimes they include a three-page auto-psychoanalysis, sometimes it's only a "Can we talk about this next week".