I'm due for a chest X-ray retest soon, and I'm really anxious about how to make sure my lungs show up clear. Do you have any advice or tips on preparing for it, or things I should do to improve my chances of a clear X-ray?

Infections seen on the chest x-ray will resolve once the infections come under control. Scars due to cigarette smoking won't resolve always but some can reduce after stopping smoking and exercising thoroughly. Keep yourself well hydrated. This isn't an assured method but has shown to help in few. I would advise you to meet a radiologist.