Giant in the Playground's Increasing Size, Effective Size, Unarmed Damage, Reach thread is of some limited utility.
Minmaxboard's Grappling Handbook, Hulkamania is runnin' wild, is of somewhat more use.
From your current position, I suggest the following.
Option 1: Sorcerer
No, really. Enlarge Person as a spell gives you +5 to your grapple modifier. It comes in a cheap wand at your level. Babau Slime increases your grapple damage with a bunch of acid (scales with caster level, buy a higher CL wand). Fearsome Grapple is a level 2 spell, so 4500gp, i.e. 'you can afford it', and adds +4 to your grapple mod. Another level 2 is Balor Nimbus, 6d6 to anyone grappling you per round. Corrosive Grasp creates multiple touch attacks, so you do normal grapple damage and it sets off the touch attack for +1d8 acid each time. Fist of Stone gives +6 enh to strength and a natural slam attack, but only for a minute. Of course, timing all this short-duration buffing requires an ambush on your part, so why not buy a wand of Chameleon to help hide, or Invisibility for sneaking around.
You'll also have an Octopus familiar from Stormwrack, for another +3 to grappling.
Sorcerer avoids having to make use magic device checks to use the wands - if you have a friendly arcane caster willing to sneak along with you and buff you for your grappling ambush, then you can forgo this option entirely.
Option 2: Warblade
Non Tome of Battle classes count as half initiator level for determining eligibility for maneuvers. So a level in Warblade with your 11 other levels counts as a 6th level Warblade, meaning you can select 3rd level maneuvers. A 3rd level maneuver (it's a stance) is Crushing Weight of the Mountain. It gives you the Constrict ability. Other than a Soulmeld, this is one of very few ways to get Constrict in the game without changing shape. Constrict is amazing, because ANY TIME YOU ROLL A GRAPPLE CHECK, even to /enter/ a grapple, to do unarmed damage, when they try to escape, WHATEVER, you get to do Constrict damage. Yes, in addition to your unarmed strike, yes, in addition to entering the grapple damage, yes every time they try to escape. You just squeeze the life right out of them. Crusader provides a better chassis, and also has Crushing Weight of the Mountain, but warblade has access to Iron Heart Surge which is very barbarian.
Option 3: Totemist
At level 2 in Totemist from Magic of Incarnum, you grow an extra pair of arms and add your essentia x 2 + 4 to your grapple mod. At this level, with the Expanded Soulmeld Capacity (Totem) feat, that's +12 to grapple. Also grants extra natural attacks from the arm, and other stuff from other soulmelds (short-range teleport, Spell Resistance, Natural Armour, Fire Resistance, etc).
Doesn't really do much for you this level, though. A good choice to take after you've dipped the low-hanging fruits in Warblade and/or Sorcerer.
Feat: Scorpion's Grasp
From Sandstorm, this feat lets you basically have improved grab for unarmed strikes and one-handed weapon attacks. It lets you punch someone, grab them, succeed on a grapple attempt, punch them again, and still have the rest of your actions free to punch people. The best part of Improved Grab, however, which is grappling at a -20 to count as not grappled and building a big ball of weak enemies that you carry around with you and use as a melee weapon, is sadly absent.
Requires claws or bite, MMII. Gives Improved Grab, and the ability to fling creatures 3 sizes smaller than you far, far away. Totally awesome.
Feat: Open Lesser Chakra (Arms)
Combined with a level in Totemist to get access to Kraken Mantle, binding the Mantle to your newly opened Arms slot will give you +2 to grapple per Essentia placed in (max 3, for +6 to grapple untyped).
Item: Gloves of Titan's Grip
From MiC, 3/day activate to get +8 Enhancement to Grapple for 7 rounds. Strength boosts are usually enhancement, but nothing really enhancements grapple except Grip of Iron, so this is great. Totally great. Buy it.
Item: Scorpion Claws
From Sandstorm. Exotic weapon that grants +4 to grapple checks. Well worth it, possibly even if you're not proficient.
Scaled Horror prestige class from Savage Species requires the 'Aquatic' or 'Reptilian' subtypes. You could make an argument that as a Dragon Disciple, you're kind of doing that. If so, it picks up Improved Grab at first level. Warblade -> Scaled Horror would be a great power boost.
Black Blood Cultist from Champions of Ruin is normally great for grapplers, but you're too high level and it overlaps with Dragon Disciple, also your alignment is wrong. Avoid it entirely.
Psychic Warrior is a great class for grapplers, but at this level you get too little benefit and too few power points to be of any use. The two key powers are Grip of Iron and Expansion, but they both want Augmentation to be great. There are more Wizard/Sorc spells that benefit grappling than there are powers, so even if you grabbed some Dorjes, it wouldn't be as worth it as a Sorc dip. Check the second link for more spells/powers that help out grappling.
Aberration Blood - This feat sets you up for Inhuman Reach, a natural reach increaser. It also adds +2 to grapples. At this level, it's probably sub-par.