Some of this has been said already, but...
1. Work on left and right hand technique - this is where the rubber meets the road in terms of your tone.
2. Learn the fret board (NO CHEATING, EITHER)
3. Work on your timing using a metronome or drum machine
4. Start off learning the major scale - first a one octave scale, then move to a two octave. You will ultimately need to learn both how to move horizontally (up and down a string) as well as vertically (from one string to another and back). Learn all the various fingerings you can use to navigate the scale. Learn to sing the major scale (which helps your ear training). Say the names of the notes out loud as you play them as well as the numbers associated with each note. Play the major scale along with a metronome and lock with the clicks - slowly at first, then faster (amazingly, I find it more difficult to play it slowly than quickly). Play the major scale in every key. Make sure to start the scale with the lowest note in the scale, not with the lowest root (except in E of course, where the root is the lowest note of the scale - on a four string anyway).
5. Print the following article - make sure you have one copy to keep with your bass and look at it every time you play, especially reading again the sections that you don't yet understand. Put a copy on the back of every toilet in your house. When you go to the bathroom, you know what to do. Put a copy under your pillow at night - I'm not sure if it works, but you never know. Seriously - it is very very good.
Introduction to Chord/Scale Theory, by Jazzbo(article)
http://www.talkbass.com/forum/showthread.php?t=125519
6. Practice every day - and I mean really practice, not just goof around. Practice the stuff that is particularly challenging. It is easy to sit and play what you know. Don't take the low road and get caught in this rut as it will stunt your growth faster than anything.
7. Play in a live band setting as soon and as often as you can. You will make mistakes - if you are going to make a mistake, be sure to play it loud and with confidence. This is better than playing the right notes, but sounding tentative and timid. Over time, you will play fewer and fewer wrong notes.
8. Learn correctly - too many people want to learn a bunch of fast and flashy licks to impress someone. There is time for that, I suppose. But leave it to the professionals. Learn to groove instead.
9. Don't quit. You will get frustrated. You will get bored. You will hit a plateau. Suck it up.
10. Listen to as much music as you can from every genre you can stomach. Try playing some of it on bass.
And a bonus one: PROTECT YOUR HEARING! You may think it is cool playing loud and blowing the doors off the place. I think it is cool, too - wear earplugs. Tinninitus sucks and once your hearing is gone, its gone. Game over...
Oh, yeah and get a really good instructor too...