Well, like I mentioned in Torontogirl's thread, I wear jeans a lot, and they're the quintessential dress-up-dress-down garment. So I guess I do have a lot of twofers. Tops can be dressed up and down easily too, as can skirts...actually, come to think of it, lots of my clothes can be dressed up or down.
For me, dressing something up involves wearing it with either tighter or more structured items, and with higher heels and more makeup and possibly jewellery. For example, I might dress down a pair of jeans with a slouchy tee shirt that is semi-tucked, a cardigan, and flat boots. I'll dress up the same pair of jeans with heeled, fancier boots or dressy shoes and a sleek fitted tee shirt and blazer. Or I might wear a crisp button-front shirt with them, with or without a topper. If I'm really dressing them up, I'll go for very high heels and maybe a cuff bracelet or even a necklace (rarely).
I think that's why I have more trouble with skirts. For me, skirt hemlines matter a lot. My short skirts only really look good with flat shoes, and that always ends up as a casual look. My skirts that are longer than knee-length only look good with heels, but heels automatically make me feel dressed-up. Jeans are more versatile that way because the same pair can look good with both flats or heels.
I'm no help with dresses, because I almost never wear 'em.
ETA I find that black, navy, grey and white are the best colours for me when it comes to dressing something up. I don't know why, but to me, colours feel more casual. I'm not very adventurous that way.