DE: Getting the best out of cover

Class is in session.

To get the best out of Dark Eldar, you must understand 2 things:  Everything you have is paper thin, and everything you have is incredibly destructive.  This means that in order to play your glass cannon army effectively, you must know how to deal damage while denying damage.

Here's a cheat sheet:

Stealth is +1 to your cover save.
Shrouded is +2 to your cover save.
Skilled Rider is +1 to your cover save.
PGL and defensive grenades gives Stealth to units being shot at 8" and closer.
Well-positioned angle is +1 to your cover save, but only under certain conditions (pg.75, picture 3).
Stealth + Shrouded = +3 to your cover save.
Stealth + Shrouded + well-positioned angle = +4 to your cover save; this means that they stack.
Jink is a flat 5+ cover save.
Flat out is a flat 4+ cover save.
Most terrain is a flat 5+ cover save.
Ruins is a flat 4+ cover save.  Jink, Flat out, and all terrain cover do not stack.

The thing to take away from this is that there are some things that stack and some that don't.  Jink is the same as 5+ cover from terrain, so you can only use one or the other.  The same applies to Ruins and Flat Out.  However, there's some things that stack on top of cover, and there's a lot of different combinations that you can pull off to deny damage.

Check this out:

Most terrain + Stealth = 4+ cover save.
Most terrain + Shrouded = 3+ cover save.
Most terrain + Stealth + Shrouded = 2+ cover save.
Ruins + Stealth = 3+ cover save.
Ruins + Shrouded = 2+ cover save.
Most terrain + well-positioned angle + Stealth = 3+ cover save.
Most terrain + well-positioned angle + Shrouded = 2+ cover save.
Jink + Stealth = 4+ cover save.
Jink + Stealth + well-positioned angle = 3+ cover save.
Jink + Shroud = 3+ cover save.
Jink + Stealth + Shrouded = 2+ cover save.
Flat out + Stealth = 3+ cover save.
Flat out + Shrouded = 2+ cover save.
Flat out + Stealth + well-positioned angle = 2+ cover save.
...etc

If Night Fighting is in effect, you gain Shrouded from 24-36" (and Stealth at 12-24").  This is Lance engagement range.  I repeat:  This is lance engagement range.

So which ones out of this list is most common for Dark Eldar?

• If I want to stay at max Lance range and engage the enemy under Night Fighting, I make sure I get a 3+ cover save (Jink + Shrouded).  Remember to always keep moving.  Ravagers can move 12" and shoot all their Lances, so make sure you draw units out of cover and Focus Fire what you can.
• If I want to relocate my units under Night Fighting instead of shooting, I Flat Out and keep either Shrouded or Stealth.  If I'm going for an extreme angle shot or a flank shot next turn, I'll just move parallel across my battle line.  This will give me a 2+ cover under night conditions.
• If I'm driving my units close to the enemy, I'll often times ignore the fact that I'm going Flat Out (obviously, because you're going fast), drive my boat behind some ruins hopefully, and angle it from the most dangerous threat (the one that's going to be shooting at your boat) to get +1 cover.  Remember that Ruins and Flat Out do not stack, but being behind some ruins allows you to angle your boat from enemy fire.  If I'm within 12-24", I'll gain Stealth and thus, I'll have a 2+ cover under those conditions.  This means that under the cover of night, you can move units incredibly far distances and be dangerously close to your opponent, 
• If I'm going second under Night Fighting, I'm either going to be out of 36" threat range completely, limit my opponents AT fire with Night Shields, or gain 3+/2+ cover from stacking terrain/ruins + Shrouded +/- Stealth + well-positioned angles. Dark Eldar have just as many options going second than they do going first under Night Fighting.  In fact, some prefer going second so they can bait enemy units into over-extending or over-committing his units out of cover instead of the first turn Alpha Strike.  Just remember that Dark Eldar see through the dark completely.
• If there is no Night Fighting in effect, you have to utilize your Night Shields, Jink and terrain to negate as much damage as possible.  Cover is now provided as long as 25% of your vehicle is obscured now, so this should be very easy obtain.  This also means that should always be moving with your vehicles, and trying to find well-positioned angles behind buildings to gain 4+/3+ cover.  Remember that Jink does not stack with most terrain, but it does stack with well-positioned angles.
• Our vehicles are designed to take advantage of extreme angles because they're long and slender, and the prows are very narrow.  Finding these angles will keep you alive under the most impossible odds while maintaining your ranged lethality.  Stare at your Raider/Venoms/Ravagers with Pg. 75 open in your rulebook.  Burn that image in your head and practice angling your ships against enemy fire until you get it right.

Updated 7-15-12, added 3 diagrams.
I did these really quick in Vassal, so excuse the rush job.

The blue arrow symbolizes the trajectory that the Ravager moved in before he engages his target (green).  The red arrows symbolize the marines that want to engage the Ravager, but the poor angle of shot will provide the Ravager with a +1 cover save behind the ruins for total of 3+ cover save.  Assume the position the melta marines are in now is the position they'll be next turn.

The gunboat advances in a direction that faces the marines with the meltagun in front.  However, the only thing they can barely draw their eyes on is the side of the Raider.  Due to the fact that the armor side they're engaging is not the same as what they can actually engage, you get +1 cover from the extreme angled shot.  The gunboat is now freely able to engage the Terminators in green.  Again, assuming the marines have moved into position already.

The Venom here did not move, he just merely changed to fully hide his left side to the Predator because of the height of the oil barrel.  The only thing that the Predator can see is the tip of the Venom and the edge of the tail.  Because the Predator cannot engage the side armor of the vehicle, the Venom gains a +1 cover save.  The Venom is now free to engage the marines out of cover in front of him while denying the Predator of his prey.

The key thing to remember about angling your vehicles is this:

• If he's facing a direction that'll give him front, side or rear armor, but he can't see your front, side of rear armor so he shoots at another armor facing [noun] at the time of the engagement, you get +1 cover save.  That's RAW.
• For RAI, casual play and speed, if you get down to the attacking model's point of view and you can barely get a piece of the target, you will probably get a +1 cover save.  Check with your opponent just to make sure.

That is all.  Class dismissed!