ArenaNet hasn't specified any criteria for entry, or even how many players will get in, but they have said that:
1. signing up faster does not increase your odds of success
2. they're selecting testers based on development needs
I'm guessing that if you have an unusual hardware configuration, or something particularly high- or low-end, that'll increase your odds. Also, if you've never played an MMO before, or aren't really a serious gamer, that might help. Although keep in mind that this is just speculation on my behalf and I really have no idea what they're looking for.