The biggest don't I've found: don't yell at people, tell them they're wrong, or say their code is terrible (even if it is). Do bring people along when educating them about accessibility. Try to find something they did well and lead with that–it will soften the blow when you point out what could be improved.
Two effective approaches I've found to creating empathy are 1) making it about people with disabilities by showing examples of how someone is impacted, and 2) appealing to their own sense of humanity with a bit of "selfish accessibility". Make designers and developers aware that they or someone close to them could be impacted by a lack of accessibility–there is a wide spectrum of possibilities, from senior citizens to low vision to a new baby occupying one arm. The Microsoft Inlusive Design Toolkit does a good job of articulating these points: microsoft.com/en-us/design/practice