Reducing the number of partners you have reduces the likelihood of becoming infected. You may choose to have only one partner (monogamy), and if you both know your status and remain true to each other you will be safe. It is important for you to have a strong trusting relationship and to have clear boundaries (that you both agree on) with respect to third parties. Also, you need to be honest with each other – if you do stray, you have to be able to tell your partner so he/she can protect him-/herself.
Another option that has been shown to be effective in lowering risk is to reduce your number of partners generally, and to practice safer sex as consistently as possible.
Furthermore, avoid high-risk partners like sex workers and always use protection with casual partners that you have met in places such as bars and clubs.