The warning first: occasion dresses are sized for a photograph taken at the start of the evening, not for the three hours of sitting, eating and dancing that come after. Try it sitting down before you buy it. Still worth the bother, because nothing else does what a proper occasion dress does, which is let you stop thinking about what you're wearing the second you walk in.
In the shop, ignore how it looks standing still under good lighting and check the things that actually matter later. Sit in it. Reach up above your head and see what the armhole does. If there's a side split, check how high it runs when you're sitting rather than standing, because those two measurements are rarely the same thing and I have learned this the hard way on a chair that was lower than expected. Check the lining separately from the outer fabric, because a polyester satin shell over a polyester lining will cling to tights and show every seam of your underwear by nine o'clock, whereas the same dress with a proper lining or a slip of viscose underneath will sit away from the body and behave itself all night. Look at where the zip sits and whether it's hidden under a seam or just sitting there waiting to show through under flash photography. And always check the fastening can actually be managed alone, because a hook and eye at the back of the neck is a design decision made by someone who has never got dressed in a hurry.
What an occasion dress should not be worn with is anything else that's trying to be the main event. A heavily structured dress with a statement sleeve or a lot of embellishment wants a plain coat, flat or low shoes you can actually walk in, and jewellery you choose once and then forget about. Save the big earrings for the plain dresses. Where this kind of dress really earns its outing is weddings, especially the awkward outdoor ceremony followed by an indoor reception that catches people out every summer, and the winter dinners and parties where a coat is inevitable and you need something that survives being covered up for half the night without creasing into a crumpled mess.
The one mistake worth avoiding is buying for the size on the label rather than the fit on your body, particularly with anything structured through the bodice. A size up with room to breathe through a meal beats a size down you can't do the zip on by the second course, every single time.