Find the trim you love first, and then look for a fabric to match. Trims can be hard to find, with limited selection. Especially if you are shopping at local stores, you will usually have an easier time finding fabrics to match.
Snip off several inches of each trim you own, and keep it in a ziplock baggie. Carry it with you always! Trust me, you'll want to have it on hand when you find a new fabric store, or stumble across a garage sale with craft supplies.
Wash your trim before you use it. You don't want any unexpected shrinkage or fading once you've put a lot of hard work into your project. Use a lingerie bag to keep it from tangling in the wash.
Always buy more that you think you need. You can always use the extra to put another band around the sleeve, or decorate a pouch. But you really, really don't want to find out you're 10" short of what you need when you are halfway through your project.
The narrower the trim, the easier it is to sew around curves. Necklines can be especially tricky. Hemlines may look like they are cut straight across, but they often have a bit of curve to them too. Really wide trims do best on sleeves, or "clavi" (over the shoulders), or the openings of cloaks, where you can let it lie perfectly flat.