One of the things I most often find myself encouraging in my students is mindfulness. The simple act of noticing what you are doing as you play can be the best teacher you ever have. Basic building blocks of good musicianship such as tuning and tempo are nearly impossible to master unless you pay attention and listen as you play.
This, however, is a skill that takes time to learn. So how do you get started?
Record yourself! The biggest challenge to playing mindfully is that it's too much information to absorb all at once. You can peel away a layer of difficulty by taking the burden of listening off of yourself until after you've finished playing. Try choosing a small passage (30 seconds or less) of your practice material and record yourself, either on video or just with audio. Immediately after you finish playing, go back and listen to yourself.
The first few times you do this, it can be awkward, embarrassing, or downright painful! The trick is to listen for the good things you do, not just for the one or two parts where you made a mistake. Take notes as you listen, and pretend that you are a teacher listening to your own student play. Here are some questions to ask yourself as you listen:
After you've listened, return to your passage and use the observations you made to make a decision about what you want to work on first. Before you play, take a moment to remind yourself of what you heard in the recording and how you want it to sound instead. Figuring out where to start when doing detail-oriented work is challenging, and knowing something specific to focus on is immensely helpful in directing your practice.
After you've spent a while practicing in this way, you will start to notice more as you play. This can be irritating, because you begin to pick up on errors that you didn't hear before. Don't let this discourage you! Rather, enjoy the knowledge that you're becoming a more perceptive musician. Noticing areas in need of improvement is the first step in fixing them, and recording yourself will greatly improve your awareness and will help you to practice in a more mindful way.