The other issue I have with the 3rd one is you're at a pretty wide of a focal length, all the way wide at 18mm. Generally for portraits it's better to go a bit longer if you have the room to back up and get the framing you want. If you look, you can see the perspective distortion a bit between her feet and her body and her head. Also the truck body seems a bit of a distortion if you look. Some of that may be the lens, which Lightroom 4 can generally correct, but some of it is just what a wider focal length does to the perspectives.
Also, what the others said. Watch out for having the back of the camera tilted at a significant angle off of perpendicular to the ground (meaning pointing the lens up/down in relation to the ground). You should also often put your subject towards one side of the frame or the other.
Some general guidelines (rules are made to be broken, right? But you gotta know them first) for composition can be found at http://www.photographymad.com/pages/view/10-top-photography-composition-rules/
. These are by no means comprehensive, but bring up topics/ideas that you can easily do more research on. Do you have a nearby community college or camera shop? Check with them about intro to photography workshops.
Don't let all of the above discourage you, you have a great start and obviously a willing model. As long as she can be patient with you, you'll get better and better! Really, only way you get better is shooting, looking at your photos, and getting feedback like from us. I look forward to seeing more!