Effective practice isn't just a physical exercise: most of the effectiveness it's in the mind too.

From my point of view, is really important to understand that playing and practicing guitar is not the same thing.

When you are playing the guitar, your goal is to have fun and entertain people.

When you are practicing guitar, your only goal is to improve some aspect of your playing.

So, there is a big difference between playing for 3 hours and practicing for 3 hours.

My own suggestions to perform a productive practice

Define your own goals

When you practice, you need to set specific goals.
So, take time to plan what you would like to accomplish during your practice session, for example master a particularly hard passage or memorize some bars of a new piece.

Set time limits and plans practice

I suggest to limit the amount of time for achieving the goal/s: this not only boosts productivity, it will, in the long run, save you time.

You can also plan your practice using Pomodoro technique.

Record your practice sessions

Really important! I suggest to all my students (experts and beginners) to record all practice sessions (audio or video), in order to perform a "post-session assessment" and plan a new practice session focused on discovered weaknessess.

Whether it is what you worked on in your session or whether it’s a repertoire piece that is part of your current program, switching from practice mode to performance mode is essential if you indeed want to feel like you are training to perform.

Furthermore, it's really useful watch/listen old recordings in order to understand the improvements made.

Keep a practice log

Every take notes during your practice sessions.
Any activity that you track creates creating a system of self accountability which propels you forward in your growth, resulting in a higher return on your time invested.

This can be very easily accomplished in the music practice with a practice log: a sheet of paper that track your practice sessions including the time, date and a tally of how many times or how much time you have spent practicing.

On Oneminutemusiclesson.com i've found a very useful template for practice log: