How to prevent collapsing nostrils?

Get evaluated. . There are many causes for nasal collapse such as trauma, congenital, and post surgical. You should be evaluated by a experienced rhinoplasty surgeon. There is nothing that you can do except protecting your nose from injury.
Support is key. The collapse is due to lack of support caused by problems with the cartilage or excessive scar overpowering the cartilage. Options include external support (breathe right nasal strips) or internal splints (available commercially in a variety of forms) or via surgical support with cartilage grafts.

Related Questions

How can I prevent collapsing nostrils etc?

Nostril collapse. Nostril collapse is most often secondary to over resection of cartilage during surgery. If this is what you are experiencing, additional nasal reconstruction can help you. Read more...