Create a checklist, and follow it. A pre-made checklist could be used, but a self-made one allows you to go through the planning process yourself. Try to arrange your equipment well in advance before you go diving. It is recommended that you do this directly after a dive so that your needs are still fresh in your mind. These needs can turn into big problems if not resolved! Plus, you will always be ready for the next dive. Read the checklist again the day before the dive for that just-in-case scenario!
All equipment, from the dive hood to the booties and from the cylinder to charging the battery in your personal dive computer (PDC) and dive light, should be accounted for. Check weighting if changing from fresh water to salt water.