It's just practice and memorisation. Here is where I'm at. I've played guitar for a decade now. When I started I pretty much learned a dozen open chords and a few barre chords. From there honestly you can play 80% of the pop stuff out there so for the next few years I was learning easy songs and improving my style and technique.
After a few years I started to get stale, I knew a couple of scales and probably a few hundred chords but I wanted something different.
So I took guitar lessons. Honest to god real person lessons. Which I really would recommend to anyone. Go to your local guitarshop and inquire. My local shop charged me 8 bucks for 30 mins. Every week I ended up spending about an hour (he always let me go over :) with a guy who taught me about a dozen licks. I did that for a few months and when it got to the point where we were trading licks back and forth I stopped. But just being around someone whose style was different opened my mind to many more possibilities.
I had wanted to learn some music theory during these lessons, but my teacher was about as knowledgable as I was. So for the next few years I still was empty in that respect but now I was getting to the point to where I could subconciously figure out what scale I was in. I got to where I could do little lead licks and chord variations that would sound good on the fly. If I played a few notes of a scale I could complete it by ear (and my fingers usually picked the right note every time)
So this is kind of disturbing, when your body does something without your concious knowledge it picks at you. So the last few years I've been reading up here and there about chord theory and scale theory etc.
I got involved with a band about a year ago, and learning to play and sing along with about 40 songs really pumped up my skills.
And then recently I noticed myself going stale again. So I opened up my website and that's when I've been pouring over the theory again. Finally a couple of days ago it all clicked and now I understand where the chords come from etc.
I'm really not sure how to figure out the key yet. But I still have so much to absorb I'm not worried about it.
I go over it all at work every night. Somewhere in a bar in Texas you'll find a bouncer writing out scales and chords on a napkin next to the door. I go over what scales I know, what scales I can derive from those, what chords I know. Where those chords are about the neck.
I look for patterns which I don't know that well. Recently I found out how to do an A chord like this : 54222x
which is a bit of a pain, but changes the voicing of the chord and makes it sound a little more bassier. This pattern is just a G chord moved up two frets. I also write out any chords/patterns I don't know the name of. Like the Cmaj7 I learned the name of the other day.
Then I'm also learning why we call chords what we call them. For example it isn't that hard to see what's happening during sus2/4 chords. (1-2-5/1-4-5) You can see the 2 and 4 right in the pattern.
There is so much to learn it will take my lifetime just to learn what there is to learn! And practice practice practice. Honestly I think it's more determining where you need help. Right now I'm working on my right hand picking technique, strengthening my hands for barre chord songs (they are out of shape and cramp up during a four minute song) and learning as much about theory as I can. I've also been working on memorizing every note on the fretboard.
Like the others I would recommend not wasting your money. You can find all of the information on the net. Any guitar videos I saw were only worth their licks and I would go the route of a real person because ask that dvd anything you want but it's not going to answer. There are quite a few books on guitar theory and technique that are probably worth it. But I honestly don't own any of them. I did once see a book which explained all the scales and modes quite nicely. Not something that I couldn't find on the net easily, but for a few bucks I wouldn't mind having it all nicely orginized and printed out. Unfortunatly I didn't have the money then and when I went back to the guitar store later they were out :(
Llynix -
One man, one guitar, one website, one ditty a day.