When it comes to awkward conversations, nothing beats talking to your kids about sex. Amy Lang is an expert on the subject, and she has created a website to help make this dreaded talk a lot easier.
A lot of parents go into the conversation unprepared and handle it as late as possible. Amy suggests the opposite. It's obviously good to prepare, but the reason stems from the big difference between how kids think about sex and how we do. We have a lot of history and emotions surrounding the subject, but it's a fairly straightforward process to a kid. They just need to know how it works and that it's an adult activity.
Preparing not only gives you the opportunity to figure out exactly what you will say when the time comes, but also helps remove awkward feelings on your part because you'll have literally removed that stuff from what you're going to say.
My parents taught me about sex when I was two or three years old through an old but great book called Where Did I Come From? and then answered any questions I had. I grew up never wondering how sex worked and knew how to approach it safely when the time came (seriously, no pun intended -- I just can't think of another way to put it).
So, in addition to Amy's numerous credentials, I can personally say the prepared and early approach worked well on me. But she has a number of other tips to help out parents worried about having the talk on her site, and a presentation that you can watch for free to get you started. If you're worried about broaching the subject with your kids, she's the woman you want to listen to.