From your profile: http://www.wowarmory.com/character-s...rmoon&cn=Lucen
OK, first thing that jumps out at me is your talent spec. Divinity is an incredibly weak talent (1% boost to incoming heals is not worth a talent point), Divine Sacrifice isn't that useful unless you spend the full 3 points to grab the incredibly awesome raidwall, and Crusade (in the Ret tree) is an amazing threat talent. Remember that almost everything in ICC meets the criteria for the full damage of Crusade, making it 6% total damage (and thus threat) for 3 points. Sexay.
http://www.wowhead.com/talent#sZV0tAbuMusIufdxfMzbcG is what I use and recommend. (you can go 1/2 Imp Judgements if you want, that second point is just useless filler 99.9% of the time)
I apologize if this sounds like a dumb question, but you'd be surprised how many problems it solves - do you know the 96969 rotation? The 1 point in Imp Judgements implies that you do, but just to be sure, go look it up if you don't know it. 96969 is the highest threat possible that you can generate while maintaining a 100% uptime on Holy Shield.
Also, this may not have much to do with tanking but the Lay on Hands minor glyph is very useful. The third glyph slot, nothing useful at all. (Sense Undead is amazing in ICC though!)
So yeah, gear. 26 expertise is the most important benchmark for Paladin threat, followed by strength (up to 2000) then hit and additional expertise. That being said, very few problems with sustained threat are caused by threat stats - typically the problem is poor rotation or a poor talent spec.
Also, you mention that the other tanks are out-aggroing you. Maybe they should hold back a bit if you're main tanking? You only need enough threat to out-threat the DPS, if your offtank is too busy waggling his e-peen to watch Omen then he's the one screwing up.
A Prot Paladin should generally only be using 2 pieces of the tier 10, because our 4-piece is crap. At the 251/264 gear level, the recommended setup is t10 head + shoulders, Cataclysmic chest, Kraken gloves (both available from Frosties), and Pillars of Might (the BiS legs - get them if you can afford them! Save up, if you can't!). You definitely want to keep the 2-piece.
Your gemming and enchanting needs a little work, stack more stam and armor. Grid of most socket bonuses and just stack +30 stam gems, and your cloak enchant should be +225 armor for example. The proper answer to "how much stam and armor should I have" is as much as possible.
For a weapon enchant, none of them are stellar. Blood Draining is the best for pure survival, and Mongoose is nice for a mix of threat avoidance and a bit of armor (although its proc rate got nerfed again). Blade Ward is basically Mongoose but godawful, and Accuracy is nice for threat if you really really need it (which you shouldn't).
edit: oh, and your trinkets (currently saved with the Key and UO) are very good for most of ICC. Keep a second stam trinket around for Sindra and LK.