My first suggestion is to be fearless. Our kids are like predators that smell fear on us. All your discomfort does is make them feel embarrassed and unwilling to open up. Regardless of a child's moral upbringing, curiosity and physiological urges will arrive whether we as the parents are prepared or not. So again, I say be fearless. Practice your "game face" so that you can address the topic matter of factly vs. like a lecture. Try to remain impartial and present yourself as your child's personal "wiki" source. They need to feel like no matter how crazy and even inappropriate the topic, they can come to you for a straight answer. No one benefits from our sons believing they can get a girl pregnant by sitting in a pool or our daughters thinking they oral sex isn't sex. Whatever the question, remember, if we, the parents, aren't willing to answer it, they WILL find an answer somewhere.
Secondly, give straight answers. If you have a questionable sexual past and you are afraid you may be asked a question you DON'T want to answer, prepare a response; make sure it's honest and straightforward. The last thing you need is for the truth to come out at an inopportune time and destroy your credibility. There is nothing wrong with saying that you will table your answer until your son or daughter is more mature. Straightforward also means explaining sex to your teens/tweens the way it actually is not the way you want them to envision it. There are no birds or bees involved, no matter how you spin it. Oprah had a marvelous episode that I watched with my oldest two children. This video took the pressure off of me: http://www.oprah.com/oprahshow/Dr-Laura-Berman-Helps-a-Mom-Explain-Sex-to-Her-Daughter-Video
On the same page there is a link to a parent's guide with diagrams to help with clarifying questions. Remember knowledge is power. We empower our child so that they have the wherewithal to make the best decision when faced with the inevitable.