The cardigan. There is the effortlessly cool long oversized cardigan look shown here by BCBG that is super trendy. To glam it up a bit for prom, try replacing that tie with a hip belt or a shiny satin bow. And if you are like me and prefer the more feminine and ladylike look after all, try this ultra girly cardigan with the puffy sleeves and bow by Miu Miu!
Then there is the shrug. I must say, I am loving it. Last year I bought a slightly furry black shrug from H&M and its great because it matches everything, it doesn't hide the dress/shirt underneath AND its warm. Its kind of similar to this black shrug here By Malene Birger, see what I mean about not hiding the dress underneath? And for something more dramatic and unique, try this black/white shrug by Moschino Cheap & Chic. It would go perfectly with those simple black strapless dresses I showed before. Talk about adding individuality to an outfit. 
Taking inspiration from the Teen Vogue prom spread, there are these gorgeous brocade cropped jackets by Hanii Y and Manoush. I think it adds just the perfect touch of romance and fancy to an outfit. Wow, the more I look at them, the more I really want one of these. . . . 
For something a bit more casual, there are these two cropped jackets from Bi La Li (I know, interesting name right?) and Diane von Furstenburg, which because of their relatively classic cropped cut, I think would still look great over a dress. But of course, these last four cover-ups are only appropriate for when you are outdoors.
And the BEST thing about these cover ups? You will most probably be able to wear it again!
Photo Credit: www.net-a-porter.com, www.bcbg.com