Sometimes I hit the gym after a long days work, when I didn't sleep well the past few nights. These seem to be the toughest workouts, and I find it a lot easier to "puss out" on my sets. To prevent this I get pissed off in my mind and take it out on the weights, especially the last few reps. Heavy Metal helps.
Now when I get pissed its just in my head with 34.869% in the eyes. I might just pace around a bit but I don't scream and curse at barbells (although I'm not opposed it)
BTW any metal bands besides Pantera, Metallica and Rammstein, you like that for lifting. Children Bodom is starting to get old.