Some tips I've used successfully over the years the few times I've been approached.
1. Don't look or act scared or nervous. To a cop, that is an instant tip off that you may be doing something illegal, you have a warrant, or you are in possession of narcotics. First and foremost, you need to at least appear to have some self confidence. That shows 1) that you have nothing to hide and 2) the cops might be in for a challenge if they decide to mess with you. If they think that you know that it is ok for you to be there, most cops and guards will leave you alone. It's not woth the hassle for them. If you look helpless, that makes you an easy target to the cops that usually have nothing better to do while patrolling the airport.
2. Try and look presentable. Don't look like a bum. I'm not saying you have to wear a suit. But at the same time, don't wear cutoff shorts, or a soiled tank top that looks like it hasn't been washed since 1987. Comfortable jeans (or nice shorts-depending on the weather), and a nice shirt and a well groomed appearance will be all you need. Remember, perception is reality to most folks, and cops are no exception. If you look disheveled, and like a bum, you will be treated accordingly-which usually involves chasing you off the property.
3. If the cops do approach you, don't try and scuffle you camera away. Short of pulling a gun or rocket launcher out, there is no worse action that you can do. You will look like you are trying to hide something. This is what people in possesion of narcotics usualy do. They think that by hurrying up and shoving it under the seat that the cop is not going to notice. Anytime a cop sees someone trying to stuff something out of sight in a hurried manner, you might as well just put a neon sign with a huge flashing arrow above you that asks the cops to come over and check you out. Even if you aren't hiding anything (which I assume you aren't), why attract uninvited attention?
4. Try and avoid hanging out on or near the fences as much as you can. If the location will simply not permit you to shoot without looking through or climbing over the fence, then do it, but be discreet, walk away and go back to your car, and try to act like it's no big deal. Be as casual and nonchalant as you can.
5. If you are approached, don't get belligerent. Just be calm, rational, and try and negotiate. People that argue with cops always lose by being ticketed, arrested, escorted off the property, or just getting harassed. If the cop is being a jerk, then just leave, try to make a couple of phone calls, and get names and permission that it is ok for you to be there. The airport ops manager is usually your best bet. If the cops come back, just say you spoke with so and so, and he said it was ok for me to be here. usually he;ll then leave you alone. If he persists, well he was going to bug you anyway. At that point it's just better to give up and move to another spot and try again some other time.
6. Last, but not least. Don't try and take pictures at places you know you have no business being. A little common sense, some patience, some courage, a good attitude, and a nice appearance will be all you need.