Be patient. Patience is essential to the beta process. Flaws in the software or gameplay are not discovered without time for those problems to manifest. Be a good tester and log on to WoW with the mindset that you might not be in for the smoothest run.
As a beta tester, your patience will be tried. You'll wipe, fail, screw up, die, whatever. It's a game that you're helping to make just that much better. Put aside the gung-ho group attitude and talk to your team about testing -- that helps me focus on the task at hand.
Be courteous. It will take a lot of bandwidth to download the Mists of Pandaria beta client, so no one waits the full client lightly. Players logging in to the Mists beta should expect the best of the best in terms of attitude. Pretend you're a professional and this testing is your assignment. Yes, it's an assignment you can log in and out of whenever you want, but if you think of your objective as fun inquiry, you'll get more done rather than fighting with others and squabbling over the pieces of the game and environment that is undergoing fixing or testing. Be courteous, and you'll be surrounded by other courteous testers.
Remember, you're a tester. You don't have to be in the testing mindset the whole time, of course. You're going to want to explore and see things, figure out your best routes for leveling and all that. Just remember that at some point during your testing when that box pops up asking for feedback, you should take the time to fill it in. There are going to be a lot of beta testers, but not every one will write feedback. Yours will always stand out.
Ask questions. Don't be afraid to ask questions in your group. Your relative experience level for this expansion is going to be low, so let people know that you're the inexperienced one. You'll be able to get help, since testers do want to clear the content and share their thoughts, and the best way to endear yourself to your group is by being honest.
If they don't want you around because you're new in a beta, you didn't want to be in that group anyway.
Submit feedback, even in a group. It bears repeating from above -- even in a group (especially in a group), you should report your experiences and feedback where applicable. Mists of Pandaria will be a better game if you do.
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!