Owen,
A few guidelines:
1. Do not accept mistakes in practice. Getting it right once out of 5 times does not mean you have it. Getting one measure right 10 times and missing in a phrase most every other time, is not getting it right.
2. Never practice beyond your point of control. This is extremely important and lays on the heels of #1. I generally recommend people practice with a metronome. When you do, start practice at a speed where you can easily play the exercise/etude/piece. Then, gradually speed up the metronome. Your body should develop the neural pathways necessary to "learn" the technique. If you ignore #1 above, you will be practicing mistakes and they will only be harder to unlearn later.
3. Approach the exercises from a musical perspective. Just because they are technical exercises doesn't mean you can't interject some artistry. It is possible to be artistic and metronomic at the same time! Rubato is not the only musical device at your disposal (though to hear some players one might think so...). The more artistic you make each note, the more artistic you will play all the time.
4. Practice the simplistic for perfection. The complex is just a bunch of simple things thrown together. Break down the complex into its simple components. It is perfectly acceptable to use something like the Carnival of Venice to learn to double tongue, as long as you follow #1, #2, and #3 above. Whatever you practice, make sure it's "yours" before you move on/speed up/ornament/etc.
5. Listen to any professional trumpet player doing the kind of playing you wish to do. Attend any live performance where the trumpet is being played live and talk to the artist (if you can). You have got to have some idea of what "right" is before you determine if you are accomplishing it. GET A PRIVATE TEACHER! If only for once or twice a month to see if you are on the right track. It is extremely helpful to have another pair of ears listen to you play.
Now, all these preclude that there are no serious problems with tone production (sound concept, breathing, embouchure - formation, mouthpiece pressure, etc.). That is another ball of wax entirely!
I hope this helps.
Tim Phillips