I had good luck with Old Navy and Motherhood. If you happen to purchase jeans from Old Navy, the panel can be sort of strange so get the smooth panel. The other has this weird elastic band that hits mid tummy and can be uncomfortable in your third trimester. Also, hit up ebay. I got LOTS of cute work appropriate tops. You're only going to be in the clothes for 9 months so there's no point in spending tons of money on maternity clothes (unless you're me and still in your maternity pants 2 months post-partum...)
For shirts, I got most of them from Target. The Liz Lange line is SO comfy and inexpensive. Go for the "Go Maternity" tanks for underneath your shirts so you don't flash everything to everyone. Honestly, I may never give 'em up..
It's essential to invest in a Bella Band or one of the generic versions. It really doesn't matter what brand. That way your pre-pregnancy pants go much further.
One last thing... COMFORTABLE SHOES!!! Because of swelling, I only had one comfy pair to wear once I hit 7 months and it was a b*tch. Good wide flats when your feet start to spread. Hope this helps!