My old gaming clan had a fair few people in that worked in the companies whose games we played, and from what they told us Beta testers are firstly not picked for being good gamers, they're picked for having great perception, analytical minds, and the drive to do the same thing over and over again to try and break the game, because despite what many thing, a beta isn't a demo for the game, it's a time to try and beat the sh*t out of it to see what needs fixing.
Secondly as someone mentioned above, they will start to dip into system specs for PC users, and over the next few waves they'll test various ranges of machines.
You'll probably find that console betas won't appear until they're ready to do the stress tests. They need lots of PC players to test the game because of how many different system setups there are, but a PS3 is a PS3 so most of it will be in house.
After the spec specific testers they'll invite the contest winners, community members (IF they decide to do that, only seen one game do that so far), and then finally they'll do a stress test and let everyone on for a week.
Atleast thats how i've seen it done for the ones i've beta'd.
This is exactly right. I've been in multiple betas and the best thing you can do is participate in any and everything that the developer puts out. Mind you SE might run things differently because they are a Japanese company and I've only participated in western betas, but if they follow the same process here are some tips that will get you into a beta:
1. Sign up for any and all mailing lists
2. Participate in fan site forums or official forums
3. Create your own fan site
4. Have your guild/LS/clan sign up as a group - the bigger the better!
5. Play the previous games from the developer if they are on a new generation of MMOs
6. Go to events that the game is showcased or demo'd, like comic-con or other conventions
7. Have an awesome computer
Obviously when the beta call comes out everyone will have more-or-less an equal chance at getting in, but doing the things listed above will give you those +1's you need to score an early beta slot.