One thing that helps me get out of my own head is getting on the rollers, forces me to concentrate on something and quiets the internal chatter. During carnivals I tend to spend twice as long on the rollers as on the track, including cooling down when I do a bit of a mental debrief as well. Sounds really obvious, and might be something you're already doing, but I know if I didn't do it, I'd be working myself up into a terrible state, making silly last minute changes, and psyching myself out of performing well.