Review module introduction and background sections.
Review module chapters for relevant areas.
Take outline or notes of relevant module areas. Optionally include the following:
Point of Interest notes:
What do they provide?
What do they want?
What are their driving factors?
Monster Stat Blocks, print or copy the stat blocks of all relevant monsters, or bookmark the proper sections of your monster manual.
Create likely encounter sheets.
Review previous session notes.
DM Screen and/or additions
Pen + Paper
Item 4×6 cards – These can be blank cards that you pen in item descriptions when you want to hand out loot or objective items so that you always know who is carrying what.
An excellent Reddit thread on the topic.