These are guidelines for music majors. If you play as an amateur, you have more freedom, although some of my amateur students have been very inspiring to me over the years regarding their dedication. Remember - devotion to your music looks like discipline...
First, the basic stuff – things that will get you fired in the real world if you forget them:
Show up. Sober, alert, and focused.
Have all your materials: music, instrument, practice sheet, pencil, etc.
Be prepared. Your lessons aren't random. They are part of a well planned curriculum, and we build on previous lessons. If you play a lot between lessons, but don't do your assignments, you will miss out on the whole process. Being unprepared is as bad as not showing up.
Now some less obvious ideas:
Save your practice sheets.
Record your lessons: this helps so much, I believe it should be mandatory. Soon after your lesson, review the recording and take notes. I suggest adding instructions and details to your practice sheet, using language you will still understand months after the lesson.
Stay in touch and email questions. Often a quick email question such as "how do I finger the chord in measure 36 so that it sounds connected to the previous scale?" can save you a bit of time. Also, clarify any questions about details of form in exercises.
Schedule your daily practice time. If you never have to say “sorry, I can't – I have to go practice,” there is something wrong. If you only practice “When you can get to it,” this is not going to work. Written homework is not more important than practice for your lesson.
Teach the people in your life to respect your practice time, and if you avoid doing the work, take responsibility for it.
Be realistic about social life: as a music major, the amount of work you must do is unfair compared to the number of credits you get for it. This is specially true if you have little or no previous formal training: in that case the first few semesters have very steep learning curves - not only in your guitar lessons, but in your theory classes as well.
Run a “Performance Class:” regular meetings with a group of students to play through your new music and the pieces you are preparing for a performance. Get some pointers from your teacher about the process of a group like this, but run it on your own - that way your lesson can work as a real goal for improvement.
Connect with other students: the practice room can be a lonely place, and knowing your friends are in the same boat helps. I used to schedule practice sessions with electric guitar master Joe Moghrabi – we would each go into our next-door studios, and come out in a couple of hours, have a little coffee (it was in Brazil...) and get back to work.
Don't spend too much time online, but use sites like this - get inspiration and tips. Look at other people's practice routines and schedules, and post your own.
Share your works in progress: play for your colleagues, friends, and family, even if it's not quite ready - let them enjoy your progress.
Materials you might need: instrument, sheet music, assignment sheets, pencil, recording or video equipment, mirror, good chair, room humidifier, earplugs, capos, mutes, stopwatch, timer, tuner, and metronome.