It's not always easy for a DPSer to try to anticipate which target you're going to focus your main attacks on. With tanking classes who have to shift through targets to establish aggro in AoE situations, /assist really doesn't necessarily help. And why should a DPS player know immediately what the best AoE gathering rotation for a given tank class is?
I've done a fair few dungeons lately where the tank has raged about DPS not attacking the right target but refused to mark them. It's so easy. When I'm tanking, I have the \ button next to shift as Mark Skull; it's a minute hand movement to make your DPSers' lives easier. They don't want a mob trying to murder them any more than you do.
And while we're talking about target marking, don't mark with a moon for sheeping (for example) and then blast an AoE move on the sheep and blame the mage. CC is a wonderful thing, but it's so easily broken. It's really easy to pull mobs away from a CCed baddie out of AoE range, so just do it.
Commenters on articles I've written lately have discussed target marks at length -- what's the right mark for this, for that, etc. I'd say the only standard ones are skull = kill first, cross = kill second. Moon usually means sheep, and square is for a hunter trap, but from there you're on your own. So when you're marking targets, make it clear what your marks mean. A sapped sheep in a block of ice is a waste of everyone's abilities!
2. Include your DPSers in speed checks. Don't assume they're ready, for starters. I've been the bad guy here many times myself, being a slightly impatient tank in runs, just checking the healer has mana and then running off and pulling. I've locked DPS out of the Bloodlord Mandokir fight more times than I care to mention and even once out of the Daakara fight because I hadn't noticed a hunter was mending or resurrecting a pet or a warlock was summoning a Felhunter or creating a Soulwell. This applies to some DPSers more than others, but there are a good number who need to have certain buffs and abilities completed and ready prior to a pull. There seems to have been a slight drop-off in the regularity of ready checks before bosses of late; I'm not talking about the raid pane ones, just a simple "r?" before pulling. It doesn't take much and can avoid some embarrassing fails!
On the other hand, when considering speed, some DPS have buffs or procs with time limits on them. For example, destruction warlocks have a Improved Soul Fire that significantly increases their damage if they can keep it rolling. I'm not saying you must chain pull like a maniac, but if you can keep up the pace enough that these sorts of things can keep ticking as we plow through trash, it'll make your DPSers happier. I grant you, it's a tricky balancing act, and you're never going to get it spot on every time, but bear it in mind.
3. Consider your movement. Don't move so much! Move more! This is basically a summary of my position regarding movement from a DPSer's perspective. There are some fights where it would be really really nice if the mobs weren't running around everywhere -- for example, most trash packs. I get it that the tank may need to kite a pack over to a particularly stubborn caster who's decided the healer looks particularly tasty, but as a general rule, once the mobs are gathered, try to keep them that way. It makes my AoE more effective and generally makes it easier for me to pick good targets than if they're all spread out. I get that this is a tank ideal too, but unless there's a pressing reason to move like bad stuff on the floor, then please don't.
A great example of this is Asira Dawnslayer in Hour of Twilight. She has this
stupid innovative mechanic where casters need to hide behind melee players to avoid being repeatedly Silenced. If the fight were completely stationary, that would be pretty easy, but there's a Choking Smoke Bomb that requires movement. It doesn't, however, require the tank to perform a complicated square dance. When tanking, I tend to tell the group I'm just going to backpedal her out of it so they have an easy time staying behind me. I see the timer on DBM, so I know it's coming and can be prepared, and I have decent gear. This, as a ranged caster DPSer, makes me happy. Also, when kiting bosses, take those few extra steps so the melee DPS who'll maximize their numbers by attacking from behind can do so without standing in bad.
4. Try to understand. DPSers have some awesome abilities, sure. Many of these are also available to tanks as well, like Raise Ally for example or the druid Rebirth. I'm including a lot of crowd control on this list, as really, in dungeons that's a big thing that DPSers bring to the party. But many of them have requirements -- runic power, for example. When things are going pear-shaped, try to bear that in mind. All combat resurrections, for example, have fairly chunky cooldowns, and maybe the warlock didn't happen to already have a soulstone in his bag to use, so don't rage on your DPS about their not being able to use the thing you're asking for the very instant you ask for it.
It's not possible to know the ins and outs of every class backwards, but try at least to give your DPSers some credit for knowing how their own things work. I'm sure there'll be some fun stories in the comments about DPSers who don't know how their own things work, but let's all try to play nice. This leads me into ...
5. Don't take your DPS players for granted. Good DPSers are good DPSers. The hunter who traps a distant caster without being asked. The rogue who interrupts, well, everything. The guy who is doing some crazy-high numbers on your meter but hasn't ever grabbed aggro. The shammy who flings down a Healing Rain or the druid who uses a Tranquility just when things are going horribly wrong. They're all good DPS players. The ones who are doing good DPS are also good DPSers (she says, stating the obvious).
Also, just because you've got the tank symbol next to your name (or indeed, the healer one) doesn't necessarily mean that you know better on kill orders, nasty abilities and the like. Listen to your DPSers. Thank them when they're doing something that helps you, and don't be defensive because you feel like you need to maintain an air of authority. As above, I've been the bad guy doing this as a tank before, and I've learned.
Remember, the DPSers are the ones who do the majority of the actual killing! And yes, I know, at certain levels and in certain situations, tank DPS is crazy. But still, DPS players are just as important as everyone else.
And in a similar vein to the tank post, don't assume your DPSers know the mechanics of every fight backwards, and don't rail on them for what you feel is low DPS. Maybe they're a touch below everyone else's gear level; maybe they haven't quite worked out what their best target is in that situation yet. Play nice, and we'll all get along.
Last but not least, I had a plea in the tank version of this for DPS to not roll need on tank gear without asking. The same applies here, and in the healing post, but since I wrote those two, the eminent Michael Gray has done a Debrett's-level guide to loot etiquette. I suggest reading it!