Don't be late for the report.
Iron your shirts, and (If you are of the shaving persuasion) be clean shaved.
Don't cock about with your phone at the expense of lookout/scan etc (even if it is in the cruise, on autopilot).
The occasional selfie is fine, as is a flight deck view photo. But not all the damn time. If your colleague is in the photo, get their explicit permission before putting it on social media.
Flightdeck conversation topics; no religion, politics or sex. Remember there is no need to fill a comfortable silence sometimes.
1st rule of dealing with company politics and gossip; assume that everyone in the company could eventually hear what you say about them, and adjust your conversation accordingly.
Make sure you take the time to ask your ramp/ops staff how they're doing - and more importantly, actually listen to the response.