i) people - keep notes about your interactions with each person you work with. Review them before you meet with them again. The greatest way to piss of managers and colleagues is to have a poor grasp on what they have told you or asked you. Whatever note taking method you use, reserve a page/section of each person.
ii) decision making - I find the spin-up cost of getting back into a project to be about getting back into the open questions that I have thought deeply about but not yet answered. I suggest finding a note taking style that works for recording in-progress thinking as its happening so that you can be interrupted and get back to deep thought asap. I like an outliner style with short-hand to indicate e.g. pros/cons, causes/consequences, connections etc.