This may sound like an odd one! But since I realised my passport has expired and I needed to take a new photo for my 10 year passport I have been very worried about looking my best! Not to mention that there are lots of rules about what you can and cant do. For example, you cant have hair in your face, and you cant smile.
In prep, I will be applying nude make up but with a hint of contouring. I will be taking with me a small lastolite reflector too - these are such handy things to carry with you as they are only small, and will help fill in any dark spots in your photograph. The difference in your passport photo will be immense but you can also use foil!