Students who want to improve should be practicing on their instrument(s) a minimum of 30 minutes a day, outside of rehearsal. Rehearsal is NOT practice time. If students come to rehearsal intending to learn their parts, the band will not improve. Practice is when you learn your part, rehearsal is when you learn everyone else's part. Given that criteria, how you practice is more important than how long you practice.
Every practice session should include: Long Tones Etude Study Literature Study
Long Tones: Every practice session should begin with long tones. Please turn on a drone while you do this. This will allow you to work on tone quality and interval tuning at the same time. Always focus on creating your best tone quality. If you don't know what that is, or if you don't like the sound you are making find recordings of great musicians to listen to. Their are many great examples online. If you want some ideas on where to start, ask Mr. Budge.
Etudes: An etude is simply a piece of music that is composed in order to help a musician work on a particular skill. Scales, arpeggios, lip-slurs are all examples of etudes.
How do I get better on that difficult part?: Figure out which passages of music you can play and which passages are too difficult for you. Only work on the passages that are difficult. (This will make your individual practice time more efficient.) On the passages that are too difficult for you, use "the method".
Play every pitch slowly, without regard to rhythm. Repeat this until every pitch sounds centered (brass players should be able to buzz every pitch on their mouthpiece) After every pitch is centered and produced with a great tone quality move on to step 2.
Now perform the rhythm without regard to pitch. When the rhythm and pitch are both independently strong, move to step 3.
Put the rhythm and pitches back together at a slow tempo. Gradually increase the tempo until you feel comfortable at about 110% of performance tempo.