When you take your waistcoat off, make sure you can’t see any shirt between the waistcoat and your trousers as it ruins the clean look. Raise your trousers higher!
Bring an extra shirt of the same fabric, just in case. This is huge. If the one you’re wearing is double cuff, bring an extra single cuff. On a high-pressure day like this, anything could happen; from forgetting your cufflinks, to a red wine spillage or even forgetting your shirt altogether! To be prepared is to be relaxed.