when I'm angry/curious/sad/in love/afraid/whatever of something and express that on my instrument, usually that gives me better results than picking up my guitar and just trying to pull something out of my ass.

Having an emotion in place pretty much tells me what the song is going to sound like.

It goes hand in hand with knowing what you want to do before sitting down and writing a song. Knowing what and which emotion you want to express before composing the song is a key ingredient.