Work it into your daily routine. Try to play during the same 15 minutes each day in the same place (preferably a quiet place that you feel comfortable playing in without distractions)
Try to avoid simply playing the tunes over and over again all the way through. If there is something that is giving you trouble, isolate it.
Start by tapping the beat in your foot. Keep this going for the tough part.
Take it SLOW. Play the tough part through until you get it right five times in a row. Then, speed it up just a little and play it again until you get it five times correctly. Keep speeding it up gradually until you're at performance tempo
Now, start a note or two before the tough part so that you begin to put it in context. Once you get it right with this extra bit, start the whole measure before. Keep adding notes/measures until you can play it within the whole of the piece
When learning a new piece of music:
Start by tapping the beat in your foot and keep it going. Count out loud "1, 2, 3, 4, 1, 2, 3, 4....". Clap the rhythm of the exercise while doing these.
Now, go back through and keep your foot tapping and clap the rhythm, but instead of saying "1, 2, 3, 4" say the note names.
Keep tapping your foot. Now, say the note names and finger the notes on your instrument without playing.
Now put it all back together and give playing it a shot. Take it slow to start.
When you're done practicing your assignments for band, don't be afraid to experiment on your instrument or to try to learn other songs for fun! Balancing our assignments with songs of your choice is a great way to stay motivated and still build skills that we are working on.