Practice. There's always a lot of discussion about "what" to practise, but you should also consider "how" to practise. Here are some things I've discovered to be valuable.
Be realistic about the amount of time you can practise each day and what you want to achieve.
Remember that this is a lifelong thing. You don't have to do it all by next week.
Consistency. Persistence. Motivation.
Keeping motivated is hard. Especially as most things you practise will only start to bear fruit after many months. But persistence and consistent practice is the key to achieving your goals.
One great piece of advice that keeps me motivated is to "practise what you love." We can't practise everything - focus on the things you really enjoy practising.
We are upper body athletes, and need to treat our practice sessions as seriously as any other athlete regards their training sessions.
If you are having problems, you might want to check out the Alexander Technique.
Use a Metronome
Wherever possible. (That means just about all the time). Work on your sense of time and groove as you practise other material.
Playing electric guitar "acoustically" might keep the neighbours happier, but you'll find you tend to pick harder...
About the Author: Dylan Kay is a professional guitarist and teaches guitar lessons in Auckland, New Zealand. He can be contacted through www.AucklandGuitarSchool.co.nz.