Why do I get hit by artillery?

Getting hit by artillery, can happen to every player. Experienced as well as new comers.
When it does happen, it's down to some of following reasons:
 

Crossing map on open area
When you're out in the open, enemy's artillery can most likely hit you. That's why it is a good idea to stay close to hill-sloped grounds, cliffs/rocks, houses and other pieces of solid cover, where most arties can't hit you.
If you have to cross open areas, make sure you go as close to hills and lower grounds as possible where arty might not be able to hit you.

Being predictable and go in straight lines
I think there was a guide, most likely on light tanks, showing lights going in curved lines. If you go in straight lines, you're easier to predict, for the enemy artillery. They can just aim, a bit in front of you, depending on your speed, and once they're loaded, they can fire and the likeliness, that you would be driving into the line of fire, is high.

Standing still
If you've been spotted, in the heat of a battle and there's arty in play, standing still in one position is a stupid thing to do. Because that invites arty to shoot and hit you. 

Knocking trees down/breaking destructable objects
Knocking trees down, or destroy objects indicates; here's a tank.
Staying near those things. with arty in play, allows the arty to go for you.

Taking position in a very well known location of the map
It is very tempting to stay at the same places on each side of the various maps. but also this, is going to make you an easier target for artillery.
Artyplayers will be knowing the common and most used positions on each map. That, can be quite a great reason to have a variety of postions on each side of every map. That way around, you'll decrease your chance to be an arty target.

Low healthpool
When you've taken some damage, which you will get to do, at some point during the battle, your hitpoints reach the lower end. 
Also in this case, you'll become a more inviting target, than if your hitpoints were full

Staying at the same place for too long
The use of sixth sense is also a good idea when playing against artillery. But once, or when you've grown experienced enough, you'll get a feeling about when you're spotted. Trust that feeling, and return to cover sooner, than the sixth sense bulp tells you to.
(As sixth sense tells you 3 seconds after you been spotted that you've been spotted, and not right when you've been spotted)

Being alone on the map, far way from allies
If you're being alone on the map, you also invite enemy arty to shoot and focus at you.
 

What about my own arty?

Should you be this unlucky to be hit by your own team's arty, (not denying that there are artillery players out there, who aren't sort of 'pure of mind)
it's most likely because you have been too close to the target, the allied arty have tend to fire at. Notice if there comes a circle on the ground. Arty players mark the ground, or some does with the t-marking system. That, is where they can hit. If you get into this area, once arty has fired, you can be unlucky enough, to be hit, having your crew stunned, by the arty-player's splash damage radius.

How do I then deal with it?
Ok. Now, we know the reasons. But how do we then deal and cope with the facts that we've been hit?
Well, some of the things have already been mentioned. Here, we brush up on the already mentioned and get into deeper detail what to do, in case of being hit, or even more to avoid being hit by enemy artillery

Move from cover to cover
When crossing maps moving around, it is about to know where the enemy artillery is most likely to take position. When you know that, you also know where to take cover.
Only the britsh artillry have a higher shell vilocity, which means they can hit closer to houses and other pieces of solid cover. When being on a map with a british artillery, try different things out on the maps, and notice what works in the cover-taking matter. That way around, you will know, next time, you meet a british artillery.Try to move from rock to rock, if possible or stay around just below the top of a hill-side. Here, the shell most likely will pass by you and only stun your crew.

Try to be unpredictalbe move around in curved and different lines and directions
Moving from point A to point B in the map, with arty in play, try going the longer way around. Make different routes every time. By that, you will be less predictalbe and harder to be hit

Stay away from knocked down destructable objects
Artillery players use any indication they can, to figure out where enemy tanks are being. Any knocked down trees, broken fences, houses, barrels, aircraft models, anything can be used for the arty's eagle eyed perspective to say "here's a tanker. Shoot here" To avoid this, stay away from those things. right after it have happened. And for anything in the world; do NOT take cover position here, when arty is in play. Otherwise, it will be guaranteed that you'll get hit

Find different useful postions
Having more than one position you can use has two benefits, and two reasons at least why doing it:
1) You'll spare preciuos battletime fighting on a silly position, instead of finghting the enemy. (-having more than one location of use)
2)You'll be less inviting target by enemy artillery, by having more than one location you can move to, which still could be working out for you.

Keep a low profile
The longer you stay in battle the better. Even any unicum would tell you so. Should you have low hitpoints, maybe an idea to be less eager to get kills. Shoot at the targets where you can, and relocate regularly. Make the arty think, maybe you go in one direction when spotted, count to 10, -or maybe 20, then double back and take a complete different way around

Keep moving
When postion is found, keep moving, in longer lines back and forth. By that, you make yourself a less inviting target for the enemy artillery. Keep hiding behind rocks and stay just below hillsides. Keep moving, and enemy arty will take someone else to shoot at.

Notice where on the map you are, compare to your allies
Make sure you have allies nearby. But still enough distance, so the arty won't get the chance of a bombardeer-medal ribbon, which means that they destroy two tanks in one shot.

As for own arty
No one can do anything about the scumbags and dickheads in artillery hunting you down for stats, or drown themselves when being the last targets. Not denying that such players does exist. And nothing really to do about it.
But in most cases, keep an eye on the areas arty marks. This is where their shells will land when they're loaded. This is where they can hit. Move away from those circled areas and you'll be fine.