Wear thick, old clothing you don't need. If you don't have that, a hoodie, cup, padding, and shinguards are a good place to start. Don't underestimate armor, getting hit on any uncovered flesh will at the least cause a nasty bruise. Wipe away paint if possible, then shoot the guy who turns away because he thinks you're down. YOU ARE NOT DOWN UNTIL YOU YELL THAT YOU ARE DOWN. THIS IS IMPORTANT ENOUGH FOR CAPS LOCK. Trust me. I may have only played once, but some dicks shot my visor, and did'nt hear me say I was down, so when I turned away to leave the field, they emptied their clips on the uncovered back of my neck because I had forgotten my hoodie. They also did this after me and my buddy had taken them out and turned away. Really ruined the sport for me, unless you're playing with people you can trust to maintain some kind of honor code/anti-injury policy(no headshots, offer surrender, someone has extra armor if you forget, etc.)
Really though, expect the worst to happen, stay with your group, and be ninja-like in your movement. Best advice in this post.