There are many ways, but the two ways that will garuantee a spot for you in the beta is by:
1. Purchase Bulletstorm special edition, or
2. Pre-order Gears of War 3.
Twitter! Many people are giving them away on Twitter. Follow @Xbox, @Gearsofwar3, @ugosweeps .
I won one from Xbox. Just tweet them and use the hashtag #gears3beta in your tweet. They randomly choose.