Some great advice above. I can suffer from being over eager/nervous and jittery on the start of a course too. I do get round it by getting to first lane and letting myself settle a bit, then watching the environment around me for hints on the wind. If my head is screwed on straight on the day I'll do a routine on every lane.. I'm therefore not distracted, just as Rob said. In fact I've become quite the creature of habit during a shoot!
I got back into shooting in 2011, it's the best way to forget all the rubbish during the week, because if you're truly focused on the shoot and your routine, everything else just disappears.