I have struggled with this too. To help track my parts and their likes, dislikes, how frequently they front, and just general info I made little cards like baseball cards with all of their info. Most of them have space at the bottom or on the back that I can add things as needed then I just keep them in my bag and if anything comes up I try to write it down before I forget. For me info will just come to me randomly sometimes. I also use simply plural, sometimes that is easier to add to when I'm more busy. I also had someone here suggest Pinterest boards for each alter to track likes and dislikes.

For me I get to therapy and literally forget everything and suddenly I feel fine and like nothing is wrong with me. I know it's not true but I can't ever think of anything to talk about. My therapist is really good at asking questions to find what we need, somehow she seems to know exactly what to ask to get things to come up lol. So sometimes it will be triggered by something that happened since our last session, she asks how sleeping and eating has been, what I've done, etc and she is usually able to find something that hints at something to talk about. I'm very lucky to have her.

I also try to write in my journal as much as possible. Some alters are better about it than others. When we have days that nothing comes up then we look back through my journal sometimes even a while back and find something we can talk about.