I think Jillian is spot-on about wearing your skirts a little higher, or a little lower, depending on the skirt, and the belly bands can do wonders, but I have 3 pieces of advice: stretchy fabrics, open jackets and a great seamstress!
I layered the stretchy tops under the jackets, so I was comfortable, but still looked professional. I purchased 2 new jackets larger than what I usually wear, and then after the pregnancy, had them altered back down to fit. Now they fit so perfectly, they look custom-made, and I didn't waste money investing in clothes I couldn't keep wearing. And I also made use of all of those pashminas. Wearing a pretty colored wrap over an otherwise boring (or snug!) top made me feel pretty while still maintaining my all-important comfort level.
One last thought... I don't think they're very trendy anymore, but when I was pregnant, those poncho tops were all the rage. They were awesome! 
Good Luck and Happy Pregnancy!