Low slung guitars may look cool however are very difficult to play.

Standing up just takes practice, I remember 15 years ago when I was first trying to play standing as opposed to sitting that it was more difficult than it looked however with time and practice, it becomes something you just do and don't even think about.

Guitar Hight:

Experiment with the strap at different lengths some where out of site of a mirror. Settle on the hight that is most comfortable and doesn't feel like your arm is being pulled off.

Oh yes, it is worth remembering that a wider strap will be more comfortable as the weight of the guitar is distributed over a wide area. For this reason the thick end of the strap attaches to the neck end of the guitar.