Become Their Source
Talk:
Have casual conversations and formal conversations. These can be everyday situations or formal/ planned conversations. These talks should be focused initially on scientific information on how their body works. Since sex is not erotic for small children, all parents really need to do with younger kids is teach the mechanics of sex. As they get older you can start to teach about other consequences of sex.
Often:
It is not enough to just have "The Talk". Your child is being educated about sex hundreds of times a day by the media, friends, and even other adults. One conversation is not going to be enough for them to have a realistic, healthy view of sex. Perpetuate future open conversations and make it easy to have a open, "no questions is a bad question" relationship.