A couple of tricks I find helped me.
On the way to the game be positive about the upcoming game, don't allow worrying or negative thoughts. "This is going to be a great game, I am going to enjoy myself"
Positive affirmations. I have a little mantra that I repeat to myself about being positive, Confident, being pro active in approach and anticipating what I need to see and where I need to be. I always end with "Today I am the ******* offspring of Collina and Webb!" - that bit is just for my own amusement! Cant take yourself to seriously! If I haven't started with this on the car journey (some bouncy music to sing along to is normal!) I will take a couple of minutes after the warm up and stretches back in the changing room prior to coming back out for kick off. A mirror really helps at this point!!
When you arrive at the venue, chat with players or team officials even the odd spectator about football - either in general or about their teams recent games. Not every one will want to chat (you can tell after the "hello, how you doing?") but there are always a few who will happily chew the fat about the beautiful game! We are all footie fans at the end of the day.
Finally, I provide a running monologue during the game "good shot, unlucky, straight out goal kick", "great tackle, won the ball clean, corner" etc. seems to help in 2 ways as the players know I am paying attention and secondly it helps me maintain concentration! I remember an assessor telling me I was too quiet, now I don't shut up!
Personal to me obviously, but they work for me.