Whether you are using a book, video, a real live human, or online lessons, keep these pointers in mind in order to get the most out our your studies.
1. Complete and master each section before moving on to the next: As you work through your studies make sure you have each new technique or idea conquered before moving on to the next. Lessons are planned to build upon each other and trying to rush through without fully understanding one will just lead to frustration and wasted efforts.
2. Study as if you were in school. Do some homework every night. If all you have is 15 minutes then use those 15 minutes. If you don't have time to read/watch and apply then do the application of your last lesson or drills such as scales and chords. Reading/watching and not having the opportunity to apply immediately will usually mean you have to relearn your lesson. Take notes. Especially if you're watching a video or working with a human. Also don't be afraid to write all over your workbooks and sheet music.
3. Apply what you've learned: Play for others. Your church, your family, that spare piano sitting in your favorite department store (ask first). Nothing drives a music lesson home better than a recital. It will also magnify what you need to work on.
These tips work whether your a child or an adult. Learning to play an instrument is a wonderful activity.