Take this pink button down shirt, for instance. It is not the worst thing I have ever seen (although it seriously needs a good press), but it isn't exactly flattering either.
I took out the collar (because I don't like wearing collars in general), shortened the sleeves, and added a bit of elastic to the front to give it a better fit. I still can't say this is my favorite maternity shirt ever, but it is much more likely to make it into the rotation (ironing really makes a difference, huh?). :)