My nose gets stuffy when I'm about to go to sleep. What can I do to help?

Saline Nasal Spray. The simplest thing to do is try some saline nasal spray before you go to bed. You can get this over the counter. If you are still having nasal stuffiness, see your local ent. You might benefit from steriod or antihistamine nasal sprays before bed. You may also benefit from a septoplasty and/or turbinate reduction.
Consider allergies. also... dust mite allergies are very common and I suggest considering environmental issues that might contribute to your symptoms. http://www.mayoclinic.org/diseases-conditions/dust-mites/basics/definition/con-20028330 http://acaai.org/allergies/types/dust-allergy.
Stuffiness. Lying down will increase blood flow to your nose, causing swelling of the lining of the nose. This will make you congested. If it bothers you, see an ent.