When doing calligraphy, I often make pencil markings on white paper, so that I can write straight and then erase the guides (I've also read you shouldn't do this, but alas). It takes time.
I usually write under a bright task lamp, so I just taped a bbq skewer underneath it, and made tick marks at the margins, 1/2 inches apart. The photos tell all.
You can do any shape guide you want with pipe cleaners and such!