Here's what I can offer...
Singing is built upon three major skills:
1. Pitch
2. Breathing
3. Diction
The problem with lessons is that most instructors try to teach vocals based of perfect Italian mouth shapes and techniques, and you end up sounding like Geoffe Tate. Even for rock, though, you have to learn the big 3.
I'd say the most overlooked of the 3 is diction. It is as crucial a component as the other two, and often problems with pitch/ intonation can be attributed to an improperly pronounced vowel. For example, singing a long "e" will almost always make you go flat.
Singing is a string of transient consonants and sustained vowels, so it stands to reason that you have to learn how to work those vowels. They are key.
Take the most overused word in rock: fire. It should be pronounced "faayaaa", not "fiiiieeerrrr".
Also, your placement should be right between your eyes, and should resonate in your head. Say the word "noon", and work with it until you can feel the "n's" vibrate your palate and your nose. (Don't sing through your nose, of course!) That's where your placement should be.
Oh man, there is just way too much to cover!!
I guess I'll just end by saying that you can definitely improve your voice. However, I'm convinced that a great voice is a gift. There is an emotional component in a great voice that cannot be learned. Here's a great comparison: Steve Perry and Steve Augeri. Both great singers with very, very similar voices. But for me Perry has that extra component. It's an element of charisma that just makes you love the voice, even when it's less than perfect. All the Beatles had it. Sting has it. Dave Matthews. Springstien, etc..etc...etc...
BTW, Matthews is another example of someone that jumps between chest voice and falsetto all the time.
Aaron
http://www.aaroncheney.com