If you aren't yet familiar with the mail art love of Japanese washi tape, let me direct you to the following resource. I like it for lots of reasons. There's no denying that the colors and patterns available are face-scrunchingly fantastic and I like that the adhesive is strong enough to hold while still being fairly repositionable.
Since I am so fond of the tape, I was a little torn when I saw this post by Between the Lines on making your own "Japanese" tape from security envelopes. I've never seen the need to make my own, because they make a security envelope pattern and because I think the tape is plenty sticky enough, but if you can't bring yourself to pay $8 for a small roll of tape, then perhaps this tutorial is for you.
P.S. Don't forget about our letter-writing social on Friday at Rare Device in San Francisco.