"You only live once" has a similar meaning to the phrases "just do it" or "carpe diem." You've got one life to live, so live right, live life to its fullest.
I took the phrase literally, though. Do we only live once? Popular science says that the human body replaces itself every 7 years or so. This isn't actually true, but many of the cells in our body have very short lifespans — stomach cells last about a week, skin cells no longer than 3 or 4 weeks, and fat cells 8 years (that explains a lot). At the cellular level at least, we constantly make ourselves anew.
For this scroll I chose a verse from the Upanishads:
"When a caterpillar has come to the end of a blade of grass, it reaches out to another blade and draws itself over to it."