If you are fortunate enough to receive a Diablo 3 beta invite, make sure you support Blizzard as much as possible.

Read quests and lore thoroughly for any spelling mistakes, investigate bugged items or quests, test out all features of the client, try out different drivers and different settings, and simply test everything you can think of.

When you submit reports, keep them clear, concise, and polite.

Providing excellent support now will increase your chances of being invited back for future beta tests. So, be extremely helpful and supportive of Blizzard and don't abuse the privilege of being selected as a beta tester.