Before you start any session you should read your notes from previous sessions. This typically takes me about 5 to 10 minutes. You don’t need the intricate detail, you’re just looking for a reminder about key points you may have forgotten.
Think about how you are going to expand on those points in your next session.
Look through your campaign module and reacquaint yourself with any location they can reach within the next four hours of play. Try and memorise as much as you can. Typically, you will get through no more than 6 encounters in a 4 hour session where an encounter includes;
A combat encounter.
A conversation with an NPC.
At certain points in the campaign that means reading about 10 pages. At other points, where choices are more varied, you could be looking at around twice that.
As an example, if you are starting out with Lost Mine of Phandelver (which I heartily recommend) you should read Act 1 – Goblin Arrows. That’s 12 pages.
Later on in the campaign, as you get to more open areas, you might need to consider that your players can go to more than one location. Work out where they can get to and read that only.
It sounds like a lot but, in practice, I can generally get away with about a dozen pages per week. If you look at the content, it’s not densely packed.