Hillclimbing is not as physically demanding as some other types of motorsport as most events only lasts a minute or so.
So extreme physical fitness is not quite so important,although the top guns all seem to be quite fit although some are not young!
I don't drink too much night before and eat calm foods,not a vindaloo curry for example!
Have a good breakfast and arrive at the venue early so there is no panic.
Prepare the car a few days before event and then check again before leaving home.
Walking the hill before the event to familiarise the track is part of the planning, even if you have been to the same venue the week before.
I think we all have little habits just before the start.
Mine are not wanting any help with harness,helmet etc,tightetning helemet strap,and harness until they hurt and then pushing all the fingers on my gloves down my fingers.
The worst thing is being red flagged and then having to sit on the start line for ages,that always results in a bad run for me as the adrenalin has built up then has nowhere to go.
hope this helps.