I have some trim in my dining room that is currently too light and want to darken it. It has a glossy finish on it and I'm not sure how to tackle darkening it. Is a good sanding enough for me to apply new stain, or is there more prep work involded?

Thanks!

Going from a lighter color to a darker is doable, but it takes a bit of trial and error to get the process and the color right. Unless the trim has been heavily sealed over the original stain, a light sanding may be all you'll need to get it to accept more stain. The more you know about the existing finish, the better the results will be: is it an oil-based or latex stain; if there is a finish coat, is it polyurethane or something else. A thin coat of shellac will take a stain, but other finishes probably won't.

The thing that is impossible to tell without trying is what color a darker stain will be once it is applied over the existing color. That is what will take the most trial and error, but if it messes up then you can always sand, prime and paint.

Good luck!

Yes sanding is in order. What kind of trim is it? Many pre-finished trims are not wood at all. The finish once sanded will wear down to the plastic underneath. Once that happens you can no longer stain it. What about painting it?
You will still need to sand anyway. But there are products that you wipe on that will etch the finish to accept paint.

All prep work when staining or painting requires a clean surface. Many products that you use to finish have their own suggestions for cleaning prior to finish. Depending on the finish your using will depend on what you clean it with. But you must always clean first before you sand. Otherwise you will sand any wax, or oils into the surface which then would make the finish not adhere properly.