When I was pregnant, I wore a back brace made for pregnant women. It held my tummy in/ up nicely, and I never got the pouch some women get after birth. I fully contribute it to that brace.
I put it on from the minute I got up out of bed, and only took it off to sleep and when I did laps in the pool. It was a pain readjusting it all the time, but now I am glad I used it. I really didn't have a choice since I had a back problem prior to getting pregnant, and with the added weight on my stomach, I would have been in serious pain if I tried to go without the brace.
Thankfully my back problem is gone now. But if I start losing weight too rapidly, I will get another brace. I figure it is like a training bra, but for my stomach, lol.