Ways to improve grapple checks
An older post of mine saved by Google:
Feats:
Improved Grapple, of course, for +4 to grapple checks.
Jotunbrud (regional, RoF) lets you count as Large for the purpose of many combat maneuvers, including grapple (+4).
Aberrant Blood and Deepspawn (LoM) both add +2 to grapple checks.
Disentangler (wild dwarf only, RoF) adds +2 to grapple checks.
Knifefighter (regional, PGtF) helps you attack with a light weapon in a grapple.
Earth's Embrace (CW) lets you eal more damage when pinning an opponent.
Savage Grapple (CAdv) lets you do sneak attack damage when grappling in wild shape.
Scorpion's Grasp (Sandstorm) lets you start a grapple when striking normally.
Multigrab and Greater Multigrab (SS, requires Improved Grab) reduces (-10)/removes the penalty for grappling with only one limb.
Oriental Adventures (3.0) has some grapple feats I believe.
Prestige Classes:
Nature's Warrior (CW) can give you +4 to grapple checks in a form with the Improved Grapple ability.
The Reaping Mauler (CW) is based around grappling. (Though you only get a +2 bonus to the actual grapple check.)
Master of Many Forms (CAdv) lets you change into big, strong forms.
Items:
Scorpion's Claws (Sandstorm, nonmagical) grant a +4 bonus to grapple checks.
Gloves of Titan's Grip (ExpPsiH) grant a +8 enhancement bonus to grapple checks (7 rounds per use, up to three uses per day, 14000 gp).
Gloves of Fearsome Grip (S&F) grant a +5 enhancement bonus for 15000 gp.
A Ring of Growth (SS) lets you grow a size category and gives you a +4 Str bonus (18000 gp).
Shapechanging:
Polymorph (or draconic polymorph from Draconomicon) lets you change into a large monster yourself. The Annis hag (Str 25) or the Cave Troll (MM3, Str 29) make good grapplers with their Strength and rake ability. For plain ol' size and Str you have War Trolls (MM3, Str 31), Firbolgs (MM2, Str 36) and Fire Giants (Str 31). (If you can change into Huge creatures there's Treants [Str 29] or Mountain Trolls [MM3, Str 36]). For non-humanoids the Darktentacles' (MM2) +16 racial bonus to grapple checks is hard to match. The Leechwalker (MM2) isn't a great grappler, but it has an ugly blood drain attack in a grapple.
For wild shape (which I'm not so familiar with) it's mostly about finding the biggest, strongest animal you can change into. Preferably with Improved Grab and/or rake attacks.
Spells:
Enlarge person (Sor/Wiz 1) increases your size.
Fearsome grapple (Sor/Wiz 2, SC) grants a +4 or +8 (CL 9+) circumstance bonus to grapple checks.
Polymorph (Sor/Wiz 4), draconic polymorph (Sor/Wiz 5, D) and shapechange (Sor/Wiz 9) can change you into something bigger and stronger.
Balor nimbus (Clr Sor/Wiz 2, SC) deals a steady 6d6/round to any creature you grapple with.
(I know I've seen other "deal damage when grappling" spells.)
Psionics:
Expansion (PsyW 1) increases your size by one or two categories.
Grip of iron (PsyW 1) gives you a +4 or better enhancement bonus to grapple checks.
Metamorphisis (Ego 4) and greater metamorphisis (Ego 9) are the psionic polymorph and shapechange powers.