This is going to sound really gay but for some odd reason I feel like giving a serious answer.
Usually I run myself through the lift cues and listen to music.
If something has shitted me off that day I'll think about that. Hating on ex girlfriends goes down quite well.
But when I'm just feeling down or I know I'm close to my limits I have this image of myself in my head as someone a lot stronger than I am now and I do my best to get there. This tends to be how I approach assessments, eating right and a lot of other things in my life.