The replies above cover pretty much everything, but I'll throw in a bit or two to help out.
Concentrate on proper Technique and Form. Punching is easy right? Not really. Most people don't know how to punch correctly, or in a way to actually maximize the amount of force generated. Get the Technique down first, everything else will come with it.
Repetition, repetition, repetition is the only way to learn how to use your off hand. You have to make a conscious decision to work your left hand. Do't tie up your right arm that's a little too drastic, and isn't accurate to simulating a fight position. Just keep your right hand glued to your cheek.
Get an instructor to watch and critique your form. A trained eye will see a lot of things you won't notice. Just a few minutes will go a long way.
You can also take a look in the I Need More Power!
thread. It has a bit of useful information and references some readily available source material.
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... As long as I don't bore you and I spark a moment of thought, my goal is achieved
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