I used thumb tacks (the ones that are tall). Worked GREAT. I would put the thumb tack in the hole in the letter position where I wanted and push it in. It also left a smaller hole than most nails would (especially the nails with the large head that fits in the special hole - insert joke here)
i actually ordered custom thin letters, painted them and mounted them on scrapbook paper in a frame. then i just hung the frame with safety hooks so it wouldn't fall.
Re: If you have wall letters. . .
My Etsy Store PIGGY'S PLACE




Same here.