1) Have Goals. If you don't have specific and measurable goals in your practice you are setting yourself up for failure and/or mediocrity. They don't need to be elaborate. Here's a few examples: "I want to be able to play through the entire song without stopping or having to look at my hands", "I want to play through this song at 80bpm", "I want to play all notes correctly", etc.
2) Work on different aspects of your playing. Once you are past the beginner level start to identify different aspects of you're playing that you want to improve on and alternate practices with these. For example, if you are a guitarist working on the lesson book and chords: Divide your time between both! This seems like a simple enough concept but many musicians (myself included!) seem to have tunnel vision when it comes to practice. It's good to work on single aspects of your playing to gain mastery, but make sure that you're not ignoring other parts.