Coughing is an important body defense mechanism of the airways. When something blocks the air passages, the body immediately attempts to expel the substance by forcing out air in the lungs under high pressure. What you have to do is get used to inhaling. The way to do this is to take a small puff, and inhale very slowly. When you start to feel the urge to cough, hold your breath and try really hard to not cough. When the feeling passes, exhale the smoke, and do it again. If you do this for just 4 or 5 puffs, trying hard to not cough, you should be able to easily inhale without coughing.