I've seen several "must-have" posts around the web, so I thought I'd throw my 2 cents in and let you know about a few things I couldn't live without!
1. Ponytail Holders. Multi-purpose! Great for keeping my hair out of my way when exercising, cooking, etc. Also great for looping through my skirt buttonhole and hooking on to the button, thus giving me a little more time in my regular clothes while looking for maternity clothes. Ghetto? Yes. Cheap? Yes. But, hey... you do what you gotta do. I refer to these as waistband extenders in public so people don't think I'm wacko. But now you know the truth.
I got a pack of Goody brand elastics at the dollar store, but you may already have some on hand.
2. Belly Band. Also know as a BeBand, or shirt extender. You can get them in a million different colors, it seems, but I recommend getting one in white or black. Great for holding up unbuttoned or unzipped bottoms, or covering up really awesome elastic waistbands. You can wear it doubled over, under your belly when it's small. Or, wear it full length when your belly starts to get huge (like mine did toward the end). As long as your hips don't change size much, you can pretty much stay in your pre-pregnancy clothes with this puppy. Also great for transitioning back into your pre-pregnancy clothes after the baby's born.
Note, this is not a pic of me... I only wish I was this thin when I was pregnant. I got my band at Target on sale for around $12.