My casual style is as simple as skinny jeans, tank top, cardigan and ballet flats. Basic makeup, blush and mascara. High pony tail and maybe a ribbon for the hair. I also used to have a hard time with casual style, especially in winter, until I just looked at what kind of winter vintage was available, then adapted it to suit my tastes.
I live in Colorado and it is COLD! So I do not wear any dresses this time of year. My uniform right now is dark wash skinny jeans with Iron Fist flats and a cute cardigan or sweater, I am always on the look out for anything cheetah for tops! On a casual day I usually have a high ponytail with a bumper bang or I put my bangs in a victory roll. I try not to go too dramatic on make up, so I will do up my eyes and just put on pink or light red lipstick. There ya go!!
Try to look for vintage style stuff at thrift shops, and I just found a couple of cute spring dresses on clearance at Forever 21. The clearance racks are the way to go right now because no one is shopping for dresses right now in the winter.
I just wear a nice pair of jeans that fit me well and also a well fitting shirt that isn't too frumpy.It may not be "retro" but it's not exactly modern either. The most important thing for me is hair. I figure if my hair is styled well, I can go casual. Also red lipstick helps too. :)