1: No obvious nudes, enough people are doing that.
1a: I like to draw her without her supernipples poking through (makes for a clean design), but considering she has impenetrable skin and thus might be tougher than the costume it might make sense. Then again, it's also a supercostume so the issue might be moot. Maybe wearing Earth fabrics there would be a noticeable bulge.
2: She must have muscles. She looks best with muscles. Just drawing her as a fit but voluptuous vixen isn't enough. Gotta draw her as a body builder. But not too much that it distracts from her style (it's not like she takes horse hormones). No veins. No. Ew. Upper body is probably going to be more cut than legs. No body-builder's balloon legs. Draw just that side of Muscle & Fitness swimmer's legs and stop. It's a balancing act.
3: She has a style. She may be Supergirl from another universe, but she's obviously trying to make herself distinct from the prime universe Supergirl. This is Supergirl without the House of L/Superman legacy to live up to. Though that may change if Superman convinces her she still has one in this universe and welcomes her into the House of El/Superfamily and she accepts. DRAAAMA!
4: No long hair. She's either gotta have between short-of-shoulder-length hair or a boy cut. Anything between including wild styles is ok since it goes with her sense of style. Bald? Bald might not work though...
5: *le sigh* The New 52 costume is the standard now, I hope they don't change it yet again... It does have a definitively Power Girl look. This is the way to draw her now and I lllllike it!
6: But she is a clothes horse. She loves new fashions. She's a great character to try new costume designs out on.
7. She's goofy. Ever since (as far as I know) the Amanda Conner era, she's gained a unique self-deprecating humor. It's ok to draw her in funny and embarrassing situations. Supergirl seems to be the straight-woman in the duo. Then again, all teens have angst these days.
7: Always remember, this is one possibility of Supergirl's future self. It's ok to draw a 10 year older future Supergirl this way. She might just become prime universe's "Power Girl". I wonder how that'll shake out as an idea? Longer hair on Supergirl then when compared to current Power Girl?
8: Ok, you've already drawn the "make a muscle" and stand around pinup versions, try to think of other more actiony poses.