Develop Good Habits. I would try to focus on improving eating habits in general. The better your nutrition, the more energy you'll have during pregnancy. The idea of gaining weight is very disconcerting for some women. However, it was my experience (and I gained a lot!) that it all comes off within a year after the baby is born. Maybe not too good for Hollywood standards, but ten years later I'm in better shape than before kids.
Article in Woman's Day. The same night I read this post, I read an article by Amy Brightfield in Woman's Day Magazine called "Foods For Fertility." According to the article, foods including slowly digested carbohydrates, protein (mostly from plant sources), good fats, and whole milk, are recommended to help improve your odds of getting pregnant. You might want to read that article for more information.
Prenatal Vitamins. I would research prenatal vitamins as well and find the best brand and best quality. Start taking those now before you get pregnant. Also, folic acid is very important for women and prior to pregnancy. Carrot juice is very good, lots of calcium.
Take those vitamins!. First, prenatal vitamins. I start taking them when I'm TTC (trying to conceive.) Second, don't stress because stress won't help. Third, I agree that starting good habits, like eating right and exercising, ahead of time is a good idea. I also agree with Lela; it does come off. Maybe not in Hollywood minutes, as she said, but within the year, it'll come. It might take a little longer but as long as you work out and try to eat well, it comes off. Lastly, there are lots of books out there. They will lay out nutritional guidelines for pregnant women. Look for one that seems to "speak" to you and your present eating lifestyle.