If you want a start with either the May 21 or 25 batches, start refreshing Blizzcon.com feverishly at around 10 a.m. and 7 p.m. PST, respectively. You'll also want to make sure you can make it to Anaheim Convention Center in Anaheim, California on Oct. 21 and October 22, otherwise, what was the point?
If all else fails, you could wait till May 28 and pay $500 to gain entry to an exclusive Blizzard charity dinner and the convention. What would the dinner conversation be like with people who have that kind of cash to drop on Blizzcon? We've got no guesses, but suffice to say we'd love to be a Darkmist Silkspinner on that wall.