A few months ago, a good friend of mine here in Costa Rica let me play his guitar. Since then I've been playing just about everyday, while reading about music theory along the way. Here are my tips to an aspiring guitarist:
1. Do the exercises in Kitharologus: The Path to Virtuosity by Ricardo Iznaola. I have yet to get to even level 1, and I have already seen great improvement in my guitar playing.
2. Learn to breath steadily and properly, this will help you in all walks of life.
3. Keep one finger on the fretboard when you can. This will allow you to play standing up without a strap, as it will give you the leverage to hold the guitar in place. Also, it allows you to keep a more accurate model in your brain of where you are on the fretboard, allowing you to take your eyes off the guitar.
[...] 2. Create a list of songs I want to learn and practice on the guitar, and print out the tabs to have at the ready by my guitar so that when I want to play I can have at the ready the materials I need to practice. I could also print out all my guitar exercises from my ancient guitar scroll. [...]