There is already excellent advice posted above. I also have some mighty frustrating days, but there are so many factors involved! Having a clean horn and a clear mind is a good place to start. I thought I'd link to my thread from a few weeks ago, when I was having a discouraging day... There were many good thoughts posted there as well.
http://www.trumpetmaster.com/vb/f131...day-77400.html (A "fighting the trumpet" day)