I agree Retin-a is great for helping to slowly shed the red marks left from acne scars, but it's a slow process and will have little-no effect on pitted scars. It also leaves your skin very red and flaky for quite a few months (make sure you moisturize, with a ph balancing, perfume-free cream).
But, the only way to get rid of (or at least greatly minimize) pitted scarring is via Carbon Dioxide Laser, Fraxel laser or a course of medium to deep peels. Dermabrasion can also work very well but it's very invasive, so I wouldn't personally recommend it unless your scarring is exceptionally bad.
I have personally had the Carbon Dioxide Laser, so I can say from experience that it really does work wonders . It gave me back my beautiful scar-less skin.
The best treatment for post-acne scarring is a topical retinoid called Retin-a (tretinoin). This will also clear any remaining active acne.
Like all topical retinoids, Retin-a can cause skin sensitivity at first, so use a bland soap-free cleanser whilst using this product.