For you guys wondering about crossing, I'll try to give you the very basic version without requiring you to take a course in botany:
When you cross a strain, the first generation of seeds you get is F1. F1 seeds are very unstable, and can show a wide variety of characteristics. The key to stabilizing it is by looking for plants with characteristics you desire, and crossing it a second time (F2), and third time (F3). It is the third generation that becomes stables and will consistently provide the characteristics you are aiming at.
Simplified example:
Let's say you want to cross an indica with a sativa to get half/half ratio. The offspring of this crop will be quite unstable. One of the seeds might be dominant indica, while another seed might be dominant sativa. Yet another seed might be a PERFECT 50/50 match that you were going for. Take that female that is the 50/50 match youre looking for, and cross it back using the healthiest male. This new batch of seeds will be F2...more stable than F1, but not stable enough to be consistent. Repeat the process once more to get a 3rd generation, F3. At this point, your seeds will be mostly stable. Some breeders go to f4 or even f5, but generally breeders prefer the F3 generation.