Or really, whenever you want to feel sexy- no matter if you're expecting or not.
Step 1: Find some really cute clothes.
I got 2 sweater/tunics from Old Navy Maternity and the rest of my clothes are garage sale/Wal-Mart maternity.
Styling really matters here. In fact a lot of the same techniques that make maternity clothes flattering also flatter bigger women as well, so read up on those! Empire waists are especially flattering. Tunics too.
Step 2: Go for layers.
Pregnancy is an odd time. I get hot/cold so often that I feel like I am either going through menopause or coming down with a flu. It's not uncommon for me to wear a t shirt, long sleeved shirt, and a jacket so I can peel off layers and start putting them back on.
Step 3: Buy new bras that fit.
Fit is the key word here. I had to use a measuring tape and several charts online to find a cup size that made sense to me. For a while, I kept getting told I was a C cup. Pfft. I have never been a C cup in my life. Finally, I found a chart that gave me a reasonable result. 46DD. I tried one on, and while it fit, there wasn't a whole lot of room for growth. I went up to a 48DD and found that not only fit me now, but there was plenty of space left in case my boobs got bigger. My boobs haven't been so supported and my shoulders didn't even feel the straps. Fit is important. (A great bra: Just My Size reasonably priced, great fit.)
Step 4: Take good care of your skin/hair
Use a nice shampoo and conditioner on your hair. I use Suave (cheap) shampoo and then Nexxus (pricey) conditioner. I only condition every other shower though, otherwise my hair gets too greasy. Skin care is also important. If you want the best bang for your buck, go with either baby oil or Udderly Smooth. While it's made for cow udders, it has so many more uses. Even night cream! Hahaha... but seriously, it works. $5 for a tub, can't beat the price and a little goes a long way. Same with baby oil. Slap it on after a shower, a dab will do you!
Step 5: Stay off the scale
Nothing says "Hello fat ass!" quite like stepping on the scale. So don't do it.
Step 6: Take lots of pictures
Even when you hate having your picture taken, take it anyway. The more I have my picture taken, the less I hate it, and even start to like some of my pictures. Not all would win a modeling call out contest, and some I would prefer never see the light of day... but still, it's important to have pictures of yourself. Don't edit yourself out of your life.
OR you can just do what I am doing now! Wear a knit tunic and a pair of underwear around the house singing "I'm Too Sexy" at the top of your lungs. To each their own :)