It can be luck of the draw, but you can also influence how much luck!
Paricipate very actively in forums such as these - not rubbish, genuine inpute as help or original critique. Write blogs. Be seen around the web.
If you have any kind of IT business, however small, you can become a Microsoft partner and contribute over on MSN and connect. Anything you can think of.
In the late days of XP, I was doing all this under another "pseudo". When the hint of "Vista" came along, I hurled email requests at all departments of Microsoft to be a Beta tester, with my background.
I think, when they get to the final selection stages, all the private "possibles" are put into a hat and a draw of some kind is conducted - but that is a wild surmise on my part.
Anyway, I was drawn and subsequently became a Beta tester for Vista and, thereafter, 7.
Great fun. Free technet subscription. You will need a lot of time (I,m retired!) so be careful.
Go for it.