They are probably using 30sta gems. And they probably utilize their professions. A tank having no professions leveled is unacceptable imo. Mining=600hP + BS=2 sockets=60sta equals minimum 1200HP you could have that other tanks probably do. Level them up or choose other profs you like and work on those.
Use a belt buckle for an extra socket.
Run TOC normal for Black Heart / take a look at other +sta trinkets.
Parry and strength gems aren't recommended. If you're really gunning for socket bonuses, dodge or expertise will probably fit better. Professions can grant you at least 40 iPoints worth of stamina(or avoidance, but let's not summon the flamers, right?). You also seem to be missing enchants, like your boots'.
I know (with regret, I wasn't playing at the time) that during the OktoberFest, a couple trinkets came out with MASSIVE stamina bonus. People who use these often have 6k more health than the normal tanks, and they really stick out. Unfortunately, there's no way to get it for a while, and the next best options are the frost badge trinket, the black heart, or some trinket that drops in... 10 man ulduar, I think?
There is always jewelcrafting. The Monarch Crab and Ruby Hare both have 62 base stamina and two gem slots. They can be gemmed to 122 stamina (each) with epic blue gems and one of them can be gemmed to 164 stamina with two of the +51 stam (JC only) gems. This would, of course, require two of your three JC only gem slots.
The Black Heart/Essence of Gossamer combo may be easier to come by though, and would be far less expensive and time consuming if you are looking at levelling jewelcrafting from the ground up and you don't have the raids on farm for the raid drops.
Black Heart and Glyph are the way to go for an "easy and good" EH trinket set, outside of raids.
The Brewfest Trinkets while they have huge stamina, have a lousy use effect.
Avoidance is mostly a "gimmick" for certain fights now, or for heroics. Most raiding is all EH now.
If you already have a max skill miner.
Your "best" profession combo is JC/BS.
From BS you get 3 extra sockets (2 specific to the BS) and from JC you get 3 VERY nice gems. With that combo you can always use your Dragon's Eyes regardless of the gear having sockets or not.
Otherwise a Miner/BS or a Miner/JC would be the way to go.
OP: Get the Shieldwarder Girdle (badge belt) or the Indestructible Plate Girdle(crafted belt) and put a belt buckle in it. You can also buy the Saronite Swordbreakers (crafted wrist) and Spiked Deathdealers (crafted boots) for more socket goodness. Also, you need a stam enchant on the boots and 2% threat on your wrists.
Use +30 stam gems everywhere unless you are socketing for +9 stam or more. Make sure you keep at least 1 purple gem for your meta. Always use dodge/stam for purple gems and defense/stam for green gems. Never use parry/stam gems.
Like already said get some stam enchants, and stack some 30+ stam gems and forget about your set bonus, only gem other stats if you are short on exp or have very low hit rating, for trinkets i recoment only to use stam once black heart and brewfest are a nice combo to begin with, the reson for this is with a high hp healers have more time to heal you up, avoidence is good but its a game of chance, and you mad get unlucky and take 2-3 hits in a row that a tank with high hp cud have toke,
general as a tank stam and armor is your bread and butter, cus it dous not rely on chance to work,
+ as a dk you get all kinds of neat little skills, like more parry with str, passive avoidence increse with new gear yay!, if you spec blood, the more hp you got the more deathstrike will heal you, and the more hp Vamp Blood will give you, and if you put 5/5 into bladet armor you can get a massiv ap buff = more threat so them damm aggro whores dont keep pulling :P