I know what you're going through. If you can, say something like, "I'm so happy for you!" Sometimes it's really hard to manage to say anything at all, so I'd prepare some kind of automatic answer like, "Oh, when are you due?" - something that you won't have to think about.
When people ask why you haven't gotten pregnant yet, you can choose one of several approaches like:
"We would like to have a child someday." and then if they ask, "Why not now?" You can say, "When we have news, we'll let you know."
"We discovered it wasn't going to be as easy as we'd expected." and then if they're nosy and ask why, you can say, "We've decided not to discuss it." or, "We found out Kevin's sperm count is nothing to write home about." or, "We're actually going for our 2nd IVF on Thursday."
Just make sure to share only what you planned on sharing. Unfortunately, infertility makes for interesting gossip... I recommend this article written by Dr. Rona Michelson about keeping your marriage together during infertility.
Good luck!
Rachel Inbar
www.fertilitystories.com