Negative intrapleural pressure is due to -
Uniform distribution of surfactant over alveoli
Negative intraalveolar pressure
Absorption by lymphatics
Presence of cartilage in the upper airway
The intrapleural pressure becomes more negative at the end of inspiration and less negative at expiration. It ranges between -3 and -6 mmHg. 
The elasticity of lung and rib cage causes it to recoil and thus creating the negative pressure. 
The lymphatic drainage of the pleural fluid also creates a negative pressure.