One of my mother friends gave me this sound advice when I was seven months pregnant, so it was too late for me. When pregnant when your regular clothes stop fitting, don't spend lots of effort making them fit and don't jump into maternity clothes. Buy some clothes two sizes bigger than you normally wear.
The big advantage to this comes a few months after giving birth. At that time, one is sick of maternity clothes, sick of looking pregnant and not yet fitting into one's regular clothes. At that point, one absolutely doesn't want to go out and pay for new "fat" clothes since one is intending to fit back into one's normal size. If one had normal (i.e. non-maternity), but larger clothes sitting around, it would be psychologically much better ("hey, I fit into some non-maternity clothes) than giving in and going and buying them new ("I can't believe I have nothing to wear so I need to go pay full price for stuff in the "women's" department.")
I'm in Colorado where it was 80 degrees on Sunday for my croquet tournament (I could still wear my maternity capris, which I love) and is currently snowing, and I'm glad I went and bought a pair of big jeans yesterday-- but I actually could have used them last March.