PRACTICE EVERYDAY! It is better to play a little bit every day rather than five hours on the weekend and not practice during the week.
Aim for at least half an hour to an hour of practice each day. Longer is definitely great, but any less and you won't see the results you want. Remember, you get what you put into the instrument.
Divide your practice day into studying different subjects. Try to focus on all aspects of your playing (chords, scales, technique, songs, etc.), not just one area.
Expand your musical horizons. Don't get locked into learning only one style of music.
Tune your guitar and warmup before you practice.
Learn and expand outside of your lesson material. Use books, videos, and supplemental lessons to expand your musical learning beyond just what you do during the lesson.
Whenever you play, whether it's practice, performance, or the lesson, the two most important things to remember: stay relaxed and comfortable.