(Believed public domain image.)
1) Always, always, ALWAYS double-check the dimensions of the stage. Why? Well last season I believed information provided by a venue - dimensions given for their own stage. As this was an out-of-state theater, I couldn't easily check so chose to believe 'em. MISTAKE. Luckily I'd designed a set where one wall flat could be omitted if necessary without killing the design. (I was a little paranoid.) In the end more than one flat had to be cut! (The design survived, luckily.) But The Venue Lied Big Time. So... double check stage dimensions.
2) Carry a sketchbook. And a pen/pencil. Don't have 'em, you'll want 'em.
3) Remember a tape measure. (See suggestions #1 and #2.)
4) Bring extra copies of the drawings. Even if you've already delivered plenty of sets. Because the theater/director/builder/client will have forgotten/misplaced/lost theirs.
Remember these four basics and... well, you'll at least feel a bit more ready to cope with the real problems.