Michael, I think the key is to make sure you have the guitar positioned well for your hands. You've probably already worked this out, consciously or not, for when you're sitting down; you may prefer a chair of a certain height (and perhaps a foot stool of some sort), a certain angle across your leg (right or left), a particular angle for your right arm to hang across the face of the guitar that allows your fingers to relax over the strings, etc. For starters, I would suggest analyzing what position works best for you sitting down, then try do duplicate it standing up. That may require some adjustment of the length of your strap, some alteration of the angle of the guitar across your body (whether parallel to the floor or more angled), and so forth. But if you pay attention to what works for you when you sit and play, you can probably identify what you need to do in order to duplicate that while standing.

A related suggestion is one that I read somehwere a long time ago that bears repeating; if you plan to perform standing, practice standing. The first time you stand and play, it's likely to feel a little awkward, and you don't want to experience that awkward moment in front of an audience. If you try to duplicate what's already comfortable (as I suggest above), you can miminize the awkwardness, but as in all things musical, it still helps to practice.