"Open Door Policy"
Establish that they can come to you for anything. A lot of parents think that the best way to control sexual behavior with their children is with formal punishments. Many times, this is a less-effective approach. If they haven’t had a chance to seriously talk with you about the subject so they have a good understanding, they will likely only see you as an irrational authoritative figure when punished or grounded. This is the best time to help them to understand why they will want to carefully consider these behaviors, not just because "I told you so." Remind them of things they might experience: pregnancy, STD’s, a bad reputation, their own personal feelings of shame and guilt, etc. You can certainly take disciplinary action if they don’t obey the rules you’ve set in place, but just keep in mind that a punishment can possibly push them further from you in being honest about the topic of sex. Make sure they understand your rules and what the consequences will be if they break them.