What's the best way to get rid of a chronic sinus infection? An MRI shows a completely clogged sinus cavity in the cheek area.

MRI=poor sinus film. Mri's are notoriously inaccurate for evaluating your sinuses, they frequently over estimate disease. If you didn't know your had sinus problems until you got your MRI and someone told you, you probably don't have a serious issue. A course of antibiotics is in order, along with follow at your local ENT to evaluate the situation.
Why is it obstructed. Try to find out what could be causing an obstruction. Is it an anatomical problem that needs to be corrected? Do you have underlying allergic problems that need to be addressed? Are you taking the prescribed medication correctly? Sinus infections take a while to resolve.