Using condoms during sex reduces risk of spread in the first place, and using a lubricant with a condom further reduces the risk (as it lessens trauma to the genital skin during sex). Silicone-based lubricants are thought to be best, but they may be less commonly available.
For people who have already been infected and get frequent flare ups, a daily dose of antiviral medication maybe taken as a preventive treatment for Genital Herpes- this can help to reduce the risk of passing it on to a sexual partner- you can discuss this option with your GP.