This. I have some amusing stories of trying to get into the US. My big tip is never travel without luggage. This sets off alarm bells.
Yep. I'll add a couple:
Never complain to the customs person about the queue
Never diss the US to them - in any way at all
Be exceedingly polite to them - use "Sir" or "Ma'am" in a non-condescending way; that'll help
Look them in the eye - they don't like shiftiness