One constant for me is a red pencil for highlighting and a black pen for marginalia. Pencils are only used if I think the pen will bleed through the paper.
Another constant is that I tend to read about five books at the same time. Different types of books being read at the same time carries these benefits: they start forming a conversation among themselves, if you are slogging through a tome like one of Charles Taylor’s big books you can take a break and read something lighter, and it is motivating to finish a bunch of books about the same time.
For many years, I have kept a log of what books I’ve read, or in some cases, reread. It serves as a good accountability to hit my goal of about 50-60 serious read books each year and it allows me to evaluate how balanced my reading is across different disciplines.
Not a constant, but close to one, is that I usually take a book with me to appointments. I don’t get perturbed when someone is late!