Especially in Los Angeles, so check SigAlert before you leave. Always leave earlier than you need to so you're not a stressball when you get to the audition. Hopefully they'll have magazines in the lobby to help you pass the time.
#4 - There will be someone in the waiting room who looks perfect for the role.
Don't fixate on them and psych yourself out before you even go in to audition! You are a talented, unique individual - the casting director will see that in the room. Keep your eyes on the prize. (Besides, that person is probably looking at you and thinking the same thing.)
#3 - You can always learn more about acting.
Is that MFA degree burning a chip into your shoulder? Get back into a class! Actors should always be developing new skills and keeping their technique fresh. If you don't want another acting class, try an improv class or a voice/movement class. A concert violinist practices every day - why shouldn't you?
#2 - Auditions and jobs will conflict with your vacation plans.
Murphy's Law, right? I once cancelled a trip to France for an opportunity that didn't even pan out - $1,000+ down the drain. But I've also turned down auditions because time with family is more important. You always try to make both work, but sometimes you just have to make a decision.
#1 - There will always be more opportunities!
There's always a feeling that each job could be your last, but there's always another audition, another job, another opportunity right around the corner. Because despite low ratings, dwindling audiences, and studio execs freaking out over falling box office numbers, the world will always be making movies, producing TV, and mounting plays and musicals. Give yourself a break if you don't get this one. There will be another. I promise.