You hit upon the problem--you have indeed, played around with contast and vibrance too much in Lightroom. Your compositions appear haphazard, unstudied, and offhand...living in California, you need to be aware that the light there is very beautiful in the evenings, but the contrast ratio of the sunlight is still pretty strong, and is really pushing the limits of digital sensors unless you use reflector fill, or fill-flash.
You're on the right track, photographing young, thin, attractive people, but my suggestion would be to slow down and really work the shots....like the girl with the football...you have simply got to get rid of that 2-liter soda bottle and the knee of the other person next to her. The background has what looks like the restrooms in the park,and people milling about.
Using a D90 and kit zooms or short wide-angle primes, you do not have a lot of control over background defocus, like you would if you had a 300 f/4 or a 70/200 2.8 zoom lens, so when shooting with the two kit zooms, which top out at f/5.6, one has to be extra-careful about backgrounds. With your 35mm 1.8 G Nikkor, you really have to watch the backgrounds, since that lens has a wide angle of view **behind** the subject!!! Anyway, slow down, and "work" the shots....like the guy using the cell phone...needs more room around him at the top,and the Dutch tilt is unneeded. Ayway, I would say to work more slowly and methodically,until you really are positive that each shot has been done as good as you can get it, and only then move on to the next subject/pose, and watch your backdrops. This style looks like you and friends went to the park with no plan, you shot 200 frames, then picked these. That's not the way to do it.
