I've been smoking cigarettes for nearly 20 years, how do I cleanse my lungs?

Stop smoking. Nothing but time will "cleanse" your lungs, meaning allowing them to heal. You can never completely undo the damage of smoking but risks of heart disease and cancer to decrease the longer it's been since you stopped. So stop smoking.
Stop smoking. Nothing you do short of stopping smoking will improve your lung health. There are many ways to stop smoking. Cold-turkey with no support system works for less than 5% of people, nicotine replacements (patches, gum etc) with a support system can help 15% of people stop smoking, and Chantix with a support system about 30%. You may actually cough more after stopping - that is your lungs cleaning out.