Dreamteam67: Very effective: Fight in Doorways, so your tanks block the path of enemies. If the Bugbears are running past your tanks, that means you have left a negotiable opening between your front liners. Close the gaps between your fighters (using 5-foot step) and choke the doorway so there is no way past. Try not to aggro a whole room of enemies, so you only have to fight a few at a time.

Sometimes works: Another trick, but not as foolproof, is to cast Grease (or Web) on the ground just in front of your tanks. Bugbears will have to run through the oil slick and save vs tripping not to fall (some will eventually get through).

Situational: Tripping is checked against strength, so high Str helps to prevent it. If you have a fighter class near the Bugbear, try executing a trip attack yourself. If the initiative order of your mage comes before the Bugbear, then the mage doesn't trigger an AoO when he gets up. Also, most monsters only get one AoO per round... so IF you manage to get the Bugbear to waste his one AoO on someone other than your tripped mage (say, by moving someone else through his ZoC), then your mage should be able to stand up without being attacked.

Very effective: Also, have multiple characters carry a supply of Charm Person scrolls in their backpacks. If you read a scroll when engaged in combat, it doesn't trigger an AoO, unlike when you cast a spell. So if/when a Bugbear breaks through to the rear, have a non-tripped character read a scroll and charm the Bugbear. Chances are your new Bugbear ally will plug the hole he ran through, preventing others from doing the same.

Less effective: Priests can cast Sanctuary (from a scroll) on a tripped mage, and that will probably protect him/her from the Bugbear's AoO when the mage stands up, but then you still have to deal with a Bugbear within melee range of your squishies.