Dungeons And Dragons Online: WotC News: Tactics and Tips: Opportunity Knocks

Wizards have the latest Tactics and Tips available on their website, and the topic is Attacks of Opportunity. Here's a quote:

Some of the less obvious attack options are still available with attacks of opportunity. For example, an opponent charges by your PC to attack the spellcaster or archer in the back rank. Use a trip attack, which may very well ensure that he doesn't get an attack off that round, and he'll provoke another attack by standing. Or perhaps a cleric with a really high Armor Class tries to cast a spell next to your character. So, grab him with a grapple check.

Some good tips on using, or avoiding, attacks of opportunity. Read the article here.