Setting a PR for each individual workout normally does the trick for me.
For instance, if I'm specifically working on muscle gain on a particular day, I'll reminisce to my last session (e.g deadlifting or squats) and try and surpass what I had done on that day. I normally do 120kg squats and 100kg deadlifts, sometimes I'll try and motivate myself to add perhaps 10-20kg more and do 2/3 more sets than my usual 10.
That's just my personal preference, but I do agree music helps a lot. I tend to go down the rock section for intensive training and jazz for cardio