Put the subject in front of a green screen, and have them not wear anything remotely green. The screen is of a colour that doesn't appear in skin tones, or hair, or what-have-you (unless you dye their hair or have body paint/make-up, but that's the exception, not the rule). The green is incredibly easy to remove from the image, or video (as it's a technique used quite often in cinematography). Bam; instantly, you have the subject and no background. You're then free to put whatever you want as a background.
Anyway, these are freaking cool. In the first, did you just use a brush with a lot of scatter, or something else? (Oh, and did you light yourself with the knowledge that you were going to do that effect, or start with even light so you could create whatever you wanted later?)