I had a denim skirt I lived in through all three pregnancies. It was by Monika Monkhouse. Wore it summer, winter and autumn.
Most useful was a pair of maternity tights (winter pregnancy). As alot of the normal tights didn't give room around the belly and itched.
I didn't buy alot of maternity tops, the most useful were the Giordano brand tops as they stretch really well and then go back really well after you have had baby.
Same goes with Metallicus.
Least useful were things that sat under my belly. Ok at the start, then when I got big they were really uncomfortable and I felt like I was always pulling them up.