The beauty of it is that the long cardigan look really IS effortless and easy to achieve. Take a plain long cardigan, like these two from UO (left) and American Apparel (right), and pair it with a tank top (black/white or more interestingly- contrasting colored!), skinny jeans (or shorts) and flats (or heels!) and you are good to go! And because the look is so plain and simple, it is also the perfect canvas to accessorize with. Throw on a sharp colored headband, a bold colored scarf, a funky long necklace, crazy earrings, chunky bangles or a fabulous bag for a touch of fun. 
If you are too lazy to even do that (like me!), then you are in luck. The long cardigan trend has been around long enough to evolve with sharp, geometric prints, contrasting colors and bold details like big gold buttons that it can be worn just as it is, sans accessories, and still look fabulous and fun. I am loving these two refreshingly colored cardigans from Marc by Marc Jacobs. The bold contrasting blue colors with the geometric prints and the gold lining (left) gives the cardigan an old fashioned yet retro look that is both refreshing and stylish. And I just LOVE the dark blue sleeves with the line of gold buttons on that pale yellow cardigan. That little details gives the otherwise ordinary yellow cardigan that extra something that makes it special. 
For more washable options, there are these classic cardigans with bold contrasting colors from American Apparel. This is the perfect way to add some color to your summer wardrobe!
For something a bit more subtle that may even last you through fall, try these cardigans with details and trimmings from Vince (left) and Urban Outfitters (right). They are far from "mumsie"- just remember not to pair them with bermuda shorts like that model above (left), it should be paired with something either really long or really short!Image Source: American Apparel, UO and Saks