1) Use a metronome.
A great musician is of no value if he or she cannot keep time. Either live or in the studio, a musician that cannot keep time will quickly find themselves on the short end of a very angry stick.
2) Slow is fast.
Many times I have seen guitarists frustrated because they are struggling with a passage at high speeds. This is a lesson learned the hard way. In my early years as a guitar payer, I would learn a part and try to play it as fast as I can. However, this is just not the right way to go about learning difficult material. You must dissect each passage and learn it inside and out. Play it slowly, focusing on technique and playing it correctly. Once you have mastered it at a slow speed is when you should increase your speed. Use a metronome to help you do this. You will discover that not only will you begin to play better, but you will be able to learn much faster.
3) Use your ears.
One of the most important concepts I like to teach my friends and students is the value of your ears. You need to develop a god ear. A well trained ear is essential when trying to play out a melody without any sheet music or tablature to help you out. To start training your ear, try learning simple songs by ear. Doing so will help you recognize chords and melodies when you hear them, and it will also help you improvise when need be.
4) Learn music theory, even if only a little bit.
A basic understanding of what makes music work will go a very long way. You should be knowledgeable of your craft. Once I started learning basic music theory, I discovered a thirst to know more and more. Knowing music theory will also help you if you suddenly find yourself being asked to play a I IV V in the key of G. You will know what that means and will know what to play without a problem.
5) Play with other musicians.
There is nothing more rewarding, both as a learning experience and a pass-time activity. Playing with other musicians is fun and helps you coordinate what you're playing. It also helps in the creative process.
6) Never stop learning.
This is one of my favourite bits of advice. The value you gain as a musician grows exponentially with the more you know about your instrument. Always continue to challenge yourself and learn new things. Learn a new chord every week, or a new scale or lick. Learn how to play blues, then learn to play jazz. Learn some new songs. Just learn something new all the time. It will make you a much better musician in the long run.
7) When you learn a song, learn the whole thing.
Every bit and piece. Learn the solo, the rhythms, and the melodies. Learn the names of the chords you are using and the scales the song uses. Learn the key. Learn as much as you possibly can about the song. Knowing it inside and out will help you memorize the song and also help you recognize elements of it in other songs.
8) Practice, practice, practice, practice.
There is no substitute for proper practice. Without practice, you will not get very far. Practice in small bite sized chunks. Spend a few minutes focusing on a particular technique. Then spend a few minutes on something else. Don't let yourself get too distracted during practice. Once you finish practicing, then you can noodle around for a while. Your brain will continue to wrap itself around the things your practice while you are doing other things. You will find that you will get better with practicing things in sections.
9) Learn the notes on your fret board.
Knowing this will open up a whole new door of possibilities. You will begin seeing the fret board in a completely different light. Patterns and scales will jump out at you. Chords will glare at you. You will be able to look at the fret board and just play what is in your head. This is one of the most rewarding things you can do for yourself.
10) Put yourself into your music.
It will sound more personal and more emotional if you can manage to put yourself into your music. This is a difficult concept to visualize, but when done correctly it really enhances your music on a different level. Don't just play the music. Put some oomph into it. Bob your head, sway with the rhythm. Make funny faces. All of this stuff sounds cheesy, but you will be surprised when you start to feel the music inside of you and the effect that has on the feel of your music. My list here could go on forever, but I feel these are some of the more important concepts and tips that I have learned over the years. Give these a try, and you will be amazed at the difference they can make after a few days of dedication. Enjoy, and keep playing those guitars!