You have already made the first step to increase your lung capacity, by quitting smoking (also, try to avoid second smoke).
It is well know that a total lung capacity depends on person's age, height, sex, weight and degree of physical activity.
Increasing the length and difficulty of your workouts is another way to increase the lungs capacity.
Remember, that you will not achieve increased lung capacity overnight, no matter what method you try.
It is good to try deep breathing techniques, water exercise (swimmers lungs will be three times better at dealing with oxygen than an average person), yoga breathing exercises, practice cardiovascular activities like aerobics, cycling, running (increased cardiovascular activity has been shown to help improve lung capacity), and if you have a chance, train at a higher altitude.
If you are already diagnosed with a chronic lung disorder, consult your physician before starting any exercise program!