Solution: Half slip skirt. Slips may sound like a thing from the past, but I seriously wear mine all. the. time. As brands are skimping more often on quality (i.e. using thinner fabrics, or only lining half of a dress), a slip can be an easy and affordable way to "rescue" a garment. I especially like them in the winter to keep unlined dresses or skirts from riding up against hosiery. I own the Vassarette 16" slip skirt (under $10!) in both nude & black. If you tend to wear shorter skirts and dresses, it's worth getting your slip skirt hemmed even shorter so there's no chance of an unsightly peek-a-boo.
Solution: A bra with convertible straps that can be criss-crossed in the back (see: convertible strap bras below), or, use a racer-back bra clip to hold your bra straps closely together in the back. You can also try using a large paperclip as a solution in a pinch.
Solution: An elastic bra-strap converter. Try one of my most popular DIY tutorials, or buy one for cheap like this 1-hook converter in black or nude (here's a wider strap version, too). Please note this solution is only for slightly lower-back garments, not ones with a dramatic dip or nothing on the back. The elastic converter can pull your regular bra band down about 2 inches in the back before the fit of the cups get affected in the front.