For me, it's the fear of uncertainty regarding the next set that gets me fired up, working with weights where I know I'm probably able to get it, but I just don't know for sure. If I know for sure that I'm going to get every rep, I don't tend to get fired up at all. I walk in the gym calmly, I do my warm ups calmly, then I pray and psyche myself up when I prep for each working set.
I used to use music to mentally pump myself up, but now I find that any music I particularly want to listen to is too much of a distraction, and music that I'm not fussy about obviously isn't going to have a huge impact on me. I don't take any supplements to get myself physically or mentally fired up -- I seem to get into the right zone mentally when needed, and I don't want to get jitters or too physically excited to focus from using stimulants. Kinda the same thing with music.