Sometimes I have to take a break, as well. As Rowuk said (and you should definitely pay attention to everything he said), we're using muscles around the lips to control the embrochure. And yes, there is definitely a decline when you miss a day. But sometimes it's just unavoidable. For instance, last semester I pulled a brutal weekend of a concert band rehearsal at midday, followed by a basketball game that night. Then another midday basketball game that Saturday, followed by a church performance on Sunday, and then a really rough rehearsal on Monday at midday (we were playing a piece that had us playing really long high notes, can't remember for the life of me what it was called). I tried playing Tuesday. Didn't happen, I was just too fatigued. Which kinda comes around to my point: like Rowuk said, we're going for a marathon kind of strength. But marathon runners don't constantly push themselves to their max limits day after day. Yes, they run everyday, but some days they push harder, and some days they cover maybe 80% of what they did the day before. That way, they still ran and exercised, but because they didn't push themselves to the absolute limit, they allowed their muscles to relax. Same deal here. As someone else said, you must practice intelligently. Hang in there, and good luck on the audition.