I am new with computer programs, however, I use Paint (very small, and simple).. Cropping I personally think does the biggest change in the picture.. When I crop my picture, it helps the viewer focus on what I want them to.. Not what their wandering eyes focus on..
Maybe cropping your pictures so their eye doesnt wander around the backround of the picture would be great.., also, try blurring the backround, maybe using a small aperture (try getting her head in a shot, with the backround NOT close nearby, so it blurrs it, but have it do something with what the picture's trying to "say" to the viewer)
You seem to be on a very good track here. But there is still room for improvement. So on that note, I'm gonna give a serious critique...in the modeling/fashion photo vein.
#1: Her shirt is too blown out, and you need a hair light top-left to fill in the shadows on her legs and immediately behind her.
#2 Just doesn't do it for me. Her skin, especially on her arms and face, is gray. Her expression doesn't really say anything...the whole pose looks fake. I also don't see any point to the odd angle. And no girl wants to look like they have giant feet.
#3 The concept is good but not executed correctly. You and she need to be dead center. Her head/shoulders/breasts/legs are all out of line with each other.
#4 Trash this one. The pose makes her look fat.
#5 Like I said before, this one is the best. Nearly perfect...but you need to rotate it slightly in order to straighten out the mirror.
I really like the lighting and color in the first and last pictures. I like the colors better in the first picture... but I love the idea of the reflection in the last photo.