Some highlights

    
Don't look at, or modify, network traffic
Don't proxy your relay exit traffic through a VPN or other proxies
Good operational security (OPSEC)
Remember that running a relay is an act of transparency (even though being a Tor user is an act of privacy), because the way to strengthen trust in relays is by having a stronger community
Be sure to set your ContactInfo to a working email address in case we need to reach you