Prepare yourself well, and get into a pre match routine. ie cards go in left pocket, whistle in right pocket, check nets, check kits etc.
Get there early and look the part...cleanly shaven, kitted out properly, wide awake etc. They'll latch onto any sign of weakness.
As mentioned above, know the LOTG inside out, and stick to them. In a difficult situation you can always refer the player/manager to the LOTG (you can't if you didn't stick to them !). County FA's can't criticise you, if you kept to the laws !
Don't talk down to players and patronise them. Just be firm and confident and show your personality.
…..just enjoy watching a football match !!!!