Well, you get a +4 for every size category you're larger than the enemy. Dire Bear is Large, right? You also get a -4 for using a light weapon (be it your unarmed strike, claw, or bite, they are all light weapons). So make a disarm attack - your BAB + the Bear's STR + -4 (Light Weapon) + 4 (Improved Disarm) + 4 (Large).