A 66-year-old man with COPD presents with chronic dyspnea on exertion and cough with sputum. He has a 45 pack-year smoking history and continues to smoke. FEV1 is 45% predicted, and he has had 2 exacerbations requiring steroids and antibiotics in the past year. Current therapy is a short-acting bronchodilator as needed. What is the most appropriate next step in maintenance therapy?