If there's one item I would tell everyone to invest their spare $$ in, it's a well-cut blazer.
They can be worn with jeans or dresses, at work or at play, day or night. So versatile! Buy yours in a good fabric and don't be afraid to part with a few more bucks than you normally would - this piece will last for years in your wardrobe so get the best you can possibly afford.
Then, like Kate Hudson here, you can just add a statement necklace, this season's oversized clutch and off to a Vogue party you trot! (Or the pub, your choice).