My definition of arty safe expands out quite a bit from just sitting next to a tall rock and gets into the lines of not so safe, but safe enough. Remember the key is to keep your presence in the battle and not just hide. You're doing no good for your team if you can't deal damage until your allies are dead and they come to mop you up.
This includes mountains, houses destructible or not, cliffs, and pretty much anything that provides at least cover from 40 to 80 degree of angle protection (0 being a straight line to your face and 90 being dropped direction down on top of you). No matter what artillery can do they can only hope to splash you but offers zero or 1/100 chance of a direct hit. This is the easiest cover to spot and use such as the bridges on port to the houses in the city on Ruinberg. It is good to use, but keep in mind that you want to still have presence in the battle. You won't have access to this at all times, if you notice most of the heavy's brawling areas are centered around locations with high cover because it negates most of the artillery interference, especially when they sit relatively still (in the same spot) to wiggle.
This is generally barriers that offer a lot of protection from 0~10 degree attacks (tank to tank) but leaves your roof relatively open such as the walls in the city on Mines or being hull down in a dip in terrain. The cover you're receiving is mostly from the front of your tank and if the artillery lobs it short they'll likely do no damage. This is very bad cover from artillery shots but a lot of times you'll be more threatened by the other tanks and will require you to be where you can reliably trade shots then trying to peek-a-boom around a corner at a much slower pace. Generally your goal if artillery is on your mind is to not be the easiest target nor the juiciest.
So there is no artillery cover, to better protect yourself you have to make shooting your tank a lower priority to the artillery by making it harder for them to hit you. If you are moving back and forth artillery is more likely to shoot the guy sitting still then you, likewise if you are on the far side of a hill from the artillery they'll have a harder time getting splash damage and will also be less likely to hurt you. However I'll go into those things as well later.
Not being spotted
And I think I should put this in as well that unless a more skilled artillery player is paying attention to tanks that are not lit on the field and purposely trying to kill you, you really don't have to worry. I've killed a few tanks by following tracers, and a few more artillery doing the same thing but it is not as common while easier targets abound.
Things you can do besides using cover!
So besides hiding behind a rock which can get you killed a lot more often then you think, mostly by flanking enemies that will either hit you in the bum, or force you to move out of arty cover to be safe from them. I've killed a good number of tanks that where being arty safe, until they pulled back a few meters because they didn't want to get shot by an allied tank.
Hitting a moving target is quite a bit harder then hitting a stationary one, especially when they are moving sideways to the arty's position and not just toward or away from them (it sucks having a target move outside of your arch and you have to turn the tank to continue following them). Now moving in a straight line at the same speed isn't that much harder to hit, especially if an allied tank doesn't track the bugger before my round lands but there are a lot more near misses from being a half second too soon / late on firing. The best way to use movement to avoid being hit is to weave a bit, turn slightly left / right and if you want to get fancy slow down or speed up. that 5 degree turn at full speed with a shell travel time of 2 seconds means my direct hit splashes no where near your tank.
Besides cover, the best kind of terrain is being on the back / far side of a hill. A good example is the dunes on Sand River. Because of the angle of the terrain an artillery has to directly hit your tank to deal damage because a meter or two short or long means that shell is exploding at the top or bottom of that hill which is no where near your tank on the middle. Likewise if your tank is on the near side of a hill, that artillery shell cannot land anywhere further away then a meter (nice splash damage range) due to it falling short or long.
Being less juicy
You really need to play some rounds in an artillery to know about this which is pretty much priority and ease of hitting. If artillery is already looking at you, they'll be more likely to continue shooting at you then the easier guy to hit on the other side of the map because of the hassle of turning the tank and re-aiming. If you are sitting behind a rock and they have nothing really easy to hit but are already looking at your direction, expect a shell to come flying in if you decide to peek around the corner to shoot someone. It is hard to describe target priority based on all the factors but I'll just make a little list of things to help you base on how likely you'll be getting shot at. Many times I've seen artillery shells fall around me to the point of laughing because I was one of the few guys lit up, probably under the least amount of cover, and even though I was dancing they just wanted me to die already!
- Is anything blocking my accuracy ring?
- Are they already in my firing arch?
- How likely are they to move before I hit them?
- Are they about to stop to take the time to zoom in to shoot?
- How much damage can I get off that shot?
- Do I need to kill them?
- Just die already!!!
- Free Kill!