Product Over Process
You're standing at the free throw line holding a basketball. Are you looking at your hands or the basket? You're waiting to bat at home plate. Are you watching the ball or your bat? In the bowling alley, are you looking at the ball or the pins? All of these situations have one thing in common: your focus and concentration is on the end result, not the process by which you are going to achieve it.
So as a musician, what is your product? How about your sound and your musicianship? These should always be the ultimate goal of any performance. Nobody has ever come up to me at the end of a concert and said "I loved how your triplets were perfectly in time" or "My favorite part was that your low D's weren't sharp." At the end of the day, what matters most is how you sounded and the musical/emotional message you conveyed to your audience.
In order to do this, of course, you need to understand what a great sound and great artistry are. The best way to do this is by spending regular time listening to world class musicians. I always ask students who their favorite players are--if they don't have any, it's clear to me that they don't spend nearly enough time listening (it's okay to have many favorite players and they can change all the time!) Once you have a great idea of what you want to sound like, then start recording yourself and listening back. Do you sound like your goal? If not, focus your practice on what needs to happen to get there.
Obviously, I'm not suggesting here that you ignore the mechanics of how to play the instrument. Imagine a quarterback working on his passing. He might make subtle changes to hand position, throwing motion, or footwork. But then, (and this is the crucial step), he practices those new techniques hundreds or thousands of times so that they become completely automatic, controlled by the unconscious part of the brain. That way, when he actually gets in the game, his attention can be solely on his targeted receiver downfield. We can do the same things as brass players. Nobody should be thinking during a performance: "Keep your finger out of the ring!"
Achieving Balance in Practice
Those people that have been on a well-coached sports team already understand this process. How many times has a soccer coach said, "Today we'll work only on penalty kicks. Tomorrow, we'll do just headers." Good coaches intuitively understand that to develop complete players, you have to practice all facets of the game, repeatedly and continuously. In fact, many elite teams use a practice model where players engage in a short drill working on one particular skill set, then run to the next drill for 10-15 minutes, then on to the next one. By keeping them constantly moving and on their toes, the coaches avoid mental fatigue and much more closely simulate the actual game situations that their players are working towards.
I think most brass players understand this concept of balance. But how many times have you found yourself playing the same Arban exercise over and over (and over and over)? How many times have you worked for an hour on the same 4 lines of an etude only to realize that you used up your entire time and never got to the rest of your practicing? I highly recommend that every brass player come up with a great fundamental routine that addresses all of the skills that you need in order to be successful (air, sound, technique, articulation, flexibilty, sight-reading, etc.) This should be the very first thing that you practice every day--and becomes even more important as you prepare for a recital, audition, or major performance. I also thing it's critical to recognize when you are mentally or physically tired and learn to take a break, or move on to something else. Continuing to practice when you are working against yourself won't really help you get better!
I've found a couple other great tricks for helping to organize and find balance in your practicing. First, keep a practice journal. Write down what you worked on, for how long, and possibly even what you achieved or learned from that practice session. By going back and looking at your journals for the past week or month, you can learn where you being efficient vs. where you might be wasting a lot of time and what you are working on a lot vs. what you are ignoring. A second trick that seems to work will is to set a timer. This works well both for people like me who often had give myself time goals in order to get all of my practicing done ("20 more minutes on my Charlier etude and I then I can watch one episode of Colbert") or for people that obsessively practice they same thing until it's perfect (once the timer goes off, you must move on).
Fundamentals Over Repertoire
This seems like a simple concept, but it's critical to brass players in every stage of their career. Think about your favorite team--how much time in their practice do they spend working on skills and drill compared to the amount of time playing scrimmages and actual game simulations. At most, it's probably 50-50. At other times in the season, practices are devoted almost exclusively to fundamentals.
The same goes for brass playing. Which do you think will lead to developing better multiple tonguing more quickly: working out of the Arban book on a variety of exercises designed to teach you to multiple tongue repeated notes, scalar passages, and arpeggios in a variety of keys, or just compulsively practicing Carnival of Venice over and over again? Learn the fundamentals first, and then apply them to repertoire.
The trap that many people fall into is, "But I have to learn my band music, so I spend most of my practice time working on that!" If you spend more of your time learning to be great on your instrument, the director/conductor will thank you later because the next piece will be so much easier to learn. Don't forget about the importance of sight-reading every day! Think about it: if you practice your fundamentals until you are a great player and learn how to sight-read anything in front of you, what piece of repertoire will present a serious challenge?
Besides the actual content, there is very little difference in the preparation and process of great athletes from great musicians. We have as much to learn from Peyton Manning and John Wooden as we do from Joe Alessi and Maurice Andre! Now go practice!