To alleviate congestion after a posterior nosebleed, follow these steps:

1. Hydrate: Drink plenty of fluids to keep nasal passages moist.
2. Humidify: Use a humidifier or take a steamy shower to moisturize the air.
3. Saline Rinse: Use a saline nasal spray or rinse to gently irrigate and clear the nasal passages.
4. Avoid Irritants: Stay away from smoke, strong odors, and allergens that can irritate the nose.
5. Rest and Elevate: Rest with your head elevated to reduce nasal pressure and promote healing.

If symptoms persist or worsen, consult a healthcare professional for further evaluation and treatment.