According to the letter DPS sent me, the following things are acceptable documentation.
- "copy of U.S. passport"
"copy of U.S. certificate of birth abroad"
"evidence of a citizenship or naturalization certificate (Alien Registration number)"
"copy of a resident alien card"
"other official documentation of citizenship"
This was on a RENEWAL, so don't assume that because you've never had to do it before, you'll be ok this time too...