The question I get asked most about shift dresses is whether they are too plain to be worth the money, since there is no shape built in to do the flattering for you. My answer is that the plainness is the point. A shift dress is cut to skim rather than cling, which means the fit has to be right from the start, because there is nowhere for it to hide a bad decision.
Where it goes wrong is when you pile on things that are also trying to make a statement. A shift dress with a busy print and a chunky statement necklace on top just becomes noise, and the same goes for a heavily textured dress under a bulky jacket, the two fight for attention and neither wins. Thick, clumpy shoes drag the whole line down too. It wants one strong accessory, maybe two, and a jacket or coat with its own clean shape, not a slouchy cardigan that undoes the structure you paid for.
For colour, I have never regretted buying a shift dress in navy, charcoal or bottle green, they do not date and they go under everything from a denim jacket in April to a wool coat in November. Black is fine but slightly less forgiving on cheap fabric, since any shine or puckering shows up more. Matte finishes wear better across the seasons than anything glossy, which tends to look like it belongs at a single Christmas do and nowhere else. In the shop, check the lining is opaque enough that you are not holding the fabric up to a window, feel for a zip that sits flat rather than puckering at the back, and look at the hem weight, because a hem that is too light will ride up the second you sit down on a bus.
A shift dress earns its stripes slowly. The fabric softens with washing, which is usually an improvement, though it can also mean the shoulders lose their structure and the whole thing starts to sit a little lower and looser than it did on day one. A good one takes that softening well and still holds its shape through the body. A poor one goes baggy at the seat within a season and starts looking like something you sleep in, which is the point you know whether you bought wisely or not.