I have a dry rash underneath my nose on my upper lip. What is the best treatment for it? It happens every few months -- goes away and comes back.

Rash. Cortisone will treat excema but not useful for underlying tinea ( fungal) or herpetic ( viral). See your dermatologist.
Topical Treatment. Most likely dry skin that has become irritated. Generally using a moisturizer and keeping your hands will help. Low dose steroid cream like Hydrocortisone could be effective.