This comes up when I practice calligraphy as well. I'm right handed and if I'm doing an LTR script (like English), I can manage fairly well without a guard sheet but if I'm doing something like Arabic, I need to be way more careful since my hand will be on top the fresh letters I write. That, apart from the lack of good instructional material, is probably why I moved away from Arabic calligraphy although I really love the form of it.