Have a structure for mentorship, for attaching new members to patient long-time contributors. These can acclimate new people to the project ecosystem and teach them proper practices.
Keep discussions in the open. If decisions are made behind closed doors and it looks like those with strong connections to project leadership have inordinately favored behavior, new members will be confused and possibly discouraged
Make sure reviews and rejections are based on issues and have clear explanations
Avoid abusive, even toxic tones in discussions, even between senior members who know each other well and may not take offense, possibly due to cultural factors. Of course, one should not go crazy and install over-bearing thought police, either internally or externally, but be sensitive. New members may not know that the person you just (humorously)insulted is your oldest friend and they may be intimidated and fear they will be treated with the same irreverence.
If you are a maintainer, don’t just exert control over submissions because you can. Lead by explaining the logic of decisions, by suggesting what would be a more acceptable change set, and by encouraging others to contribute their views rather than ruling solely by fiat.