How can you prevent scarring?

The best way to prevent any acne scars from forming is to prevent more acne in the first place. Visit a dermatologist so they may evaluate your active acne and develop a course of treatment to prevent further outbreaks.
Prescription Drugs vs. OTC
Though over the counter topical treatments do work, the prescription drugs which can be prescribed by a physician are generally much more effective than the OTC products. The physician may also determine whether you need oral antibiotics for a period of time along with topical antibiotics and/or keratolytics.
Topical Treatments
There are also a number of topical products which can help the skin improve itself and improve the collagen remodeling process. Products such as Retin-A and alpha hydroxy acid which will not only improve acne but will also help stimulate new collagen formation and improve acne scarring. Also important is a strong sunscreen to prevent any sun damage to the skin as it heals.