Volume Pedal. I couldn't play a gig without one. It gives me dynamic control as well as allowing me to go from a heavy/dirty sound to a subdued clean/semi-dirty sound without having the **click** I just changed channels sound.
I don't mean to rehash what everyone said here. There were some great tips and advice given. That being said, I would definitely learn some Periphery and Meshuggah tracks. You will begin to see patterns in their compositions. Intervals will help tremendously. Especially when it comes to...