Here are some basic rules that should keep you out of trouble:
DO NOT use, expose, or show your Airsoft guns on public land or in plain public view.
DO NOT play Airsoft games on public property such as schools, parks, or playgrounds.
DO NOT play Airsoft games on private property WITHOUT the owner's permission.
When transporting your Airsoft be sure that it is completely concealed from public view and not readily identifiable as a weapon. In the event that you are confronted by law enforcement officers comply with all their requests.
It is absolutely vital that everyone participating in Airsoft games take every precaution that they can to be safely protected. Even though Airsoft is far less dangerous then paintball the Airsoft guns still have the potential to do damage to you if you are not careful. To prevent unnecessary bodily harm while playing Airsoft games you should use goggles, long sleeve shirts, and gloves. As the saying goes, "It's better to be safe then sorry!" In addition to your personal safety remember that the future of Airsoft is in the hands of those playing it right now, and if we aren't acting responsibly then there will be no future for Airsoft in the years to come. Lord only knows how trigger happy Americas courts are in regards to this sort of thing so please be careful when you play and act like an adult whenever you are using an Airsoft gun.
EYE PROTECTION (Very Important!!!)
You should ALWAYS wear eye protection at the very least. It wouldn't be intelligent to lose the use of one or more of your eyes just to play a game now would it? I didn't think so. Make sure that you are using high endurance goggles that can withstand the impact of a plastic Airsoft BB at whatever FPS limit you have established. It's a good idea to test whatever goggles you are planning on using by shooting at them from about 5-10 feet away PRIOR to using them in a Airsoft game. This way you know for sure that they will be strong enough. Masks are even more protective then Goggles and if you don't mind wearing one then you should do so. There are a wide variety of Paintball masks on the market currently so you shouldn't have any difficulty in finding one that you like. Considering that paintballs on the average have a more powerful impact then a 6mm plastic Airsoft bb you should be safe using most of them. And finally, NEVER ever remove your eye protection while on the field or still in a game. If you are seen without your eye protection on you will be considered "killed" for purposes of the game and, depending on the hosts, may be asked to leave permanently!
Unless you are a glutton for pain you should plan on wearing a long sleeve shirt, full length pants, boots, and gloves when you play Airsoft. Although you wont be seriously hurt by getting hit by an Airsoft BB in these areas (even up close) it's still smart to cover as much of your skin as possible because taking a hit on bare skin can result in a bleeding injury. Even a long sleeve cotton shirt can greatly reduce the chance of a nasty hit happening. Ideally you'll want to get a hold of old military fatigues or BDU's. These are ideal for use in Airsoft games. Because you are usually only revealing your head/weapon/hands to the enemy you are strongly encouraged to wear gloves as well. Padded gloves are the best.
SAFE USE OF WEAPONS:
Whenever you are in the designated safe zone ALWAYS keep your Airsoft gun's safety on and it's barrel plugged if you have access to any plugs. This will prevent accidental discharges of your weapon in areas that people have their masks and protective clothing removed. If you want to test fire your weapon then do so outside of the safe-zone. Failure to comply with this rule may get you banned from a particular plying location.
I wish this didn't have to be said but I know that there is a high caliber of none responsible individuals out there in the world so here goes. DO NOT UNDER ANY CIRCUMSTANCE EVER BRANDISH AN Airsoft WEAPON IN A PUBLIC PLACE! DO NOT EVER ATTEMPT TO USE AN Airsoft WEAPON IN A THREATEN MANNER AS IF IT WERE A "REAL" WEAPON! It's already clearly stated in the US Law that if you use an Airsoft gun as if it were a real gun then you will be charged and tried as if it were actually a real gun! You have been warned! And finally, DO NOT EVER UNDER ANY CIRCUMSTANCE POINT AN Airsoft GUN AT ANYONE WITHOUT THEIR EXPRESSED CONSENT!