I'm no expert, but here are few tips I stumbled upon last year when I gave my two oldest kids the ole "birds and the bees" talk (at the time, they were 10 and 8):
1) Make sure they hear it from you first. (If you wait until junior high you've waited too long.)
2) Be detailed. Their friends will fill in the blanks that we gloss over.
3) Laugh with them. You know it's funny.
4) Keep it God-centered. Tell them sex is a gift from God to be stewarded in a godly way and for the glory of God.
5) Pray with them when the talk is finished.
This can go a long way in preserving your precious children's innocence and decrease the chances they will be swept up by our sexually idolatrous society. Plus, it makes for a hilarious family night!