On-call handoff conversations. At the end of a rotation, discuss big events that occurred, including anything trending in the wrong direction. This helps everyone get on the same page, and the early warnings are a chance to prepare in advance for possible problems.
Keep your contact information up to date. Correct contact info for your team members reduces stress on the team, especially if there is a newer team member taking on-call for the first time.
Keep playbooks where everyone can find them. This enables consistency in problem solving, improves time to resolution, and increases work/life balance considerably.