Artillery:
If you are far enough into enemy territory to actually hear the artillery go off, this can be a very good indication. The longer you play, the more likely the sound of artillery going off works as an automatic beacon for you to start checking your surroundings. Are you in an area that is most likely about to get artilleried? Are you capping a flag in an undefended base? Are you surrounded by a large number of your own team? Did you just take a base? Now that we've identified the circumstances, what do you do about them.
"I just captured a base." Leave that base immediatly! Even if you plan to defend it, leave it momentarily, let the almost 100% certain arty strike hit and then come back to defend it.
"My entire team is entrenched in this one area" Time to start moving! Try to stay on the perimeter of your team and push whichever direction you are trying to go. Worst case, at least find a building incase your whole team gets hit. In these scenarios, remember though, you will eventually get hit by 1 or 2 artilleries, usually when your team is entrenched it means you are probably winning the war though. As long as you win the war, 2 arty deaths is not a big deal.
"I'm in a house, I should be safe right?" Sort of. If it is a small hut / bunker, make sure you stay crouched or prone. If a shell hits your building directly and you are standing straight up, there is a good chance the splash damage will still cause you to die. Staying prone may just save your life. Also remember to try to stay in the middle of the room, away from the doorways and windows to better ensure your survival. Basically, just remember you aren't invincible simply because you found shelter.
"Holy crap I can't believe this entire base is undefended!" Yea, and this is right about when you get hit by an artillery. A fun tactic to do is bring the flag down to white, making the base neutral, and then run away. Doing this can set a trap for the other team coming back to recapture it as well as keeping yourself out of the artillery. If artillery does infact hit, go back in afterwards and capture. Either way, there's no such thing as a free flag.
Last but not least, be aware of sneaky commanders. The kind who would drop a supply crate near your tank, just to keep you there long enough to get hit by the artillery (or a parachute jeep if you are on foot!).
Jets:
Jets for all the Quake fans out there, are quad damage, pentagram and eyes all rolled into one. Basically, they are the unfair advantage, the ace in the hole. A good pilot has a lot of options available to him. He can mass carpet bomb bases, leaving 20 people dead in his wake, he can hound choppers all map long, he can pick off vehicles, anything from tanks to jeeps, he can swoop down and shoot people right out of AA and other turrets. But there are some things you can do to counter this.
If you are running straight and you hear a screech in the sky, move to your right or left as soon as humanly possible. If you are in a jeep or other moderately fast vehicle, this could be enough to survive a bombing, hell I've survived some on foot even this way. The main thing to remember is hearing a screetch above you is 99% a bad thing. The one or two times it might actually be your teams jet is probably when he teamkills you accidently anyways.
If you are sitting on an AA turret getting a bunch of kills, it's time to move. Jet's look at people sitting on AA like kids look at a pinata. If you really want to be good at AA, stand next to the unit but do not actually use it until a vehicle is nearby. This draws far less suspicion from radar.
If you are in a vehicle, constantly think of ways to make yourself a more difficult target. It's harder to bomb a tank sitting in the tree line than it is to bomb a tank sitting in the middle of a road.
If you are in a chopper, turn into Jets. Jet's love to slow down to take down choppers, if you turn into them, you cut down the amount of time they are behind you, which means less time to lock on and less time to fire bullets into your vehicle. Also, try to fly your chopper in odd spots. Don't fly straight from the helipad to where the enemies are because this is most likely where the jets expect you to be.
When you are on foot, you usually don't have to worry about jets too much. Just make sure you are not in a massive group of teamates or you will get bombed. It is a waste of time for a Jet to constantly try to machine gun a small squad of men. And bombing them simply wouldn't be worth it.