Garment sewing will almost always require a closure of some kind. While fabric weight, overall style of the piece and location of the closure are all determining factors, ultimately you should choose one that adds function and detail to your project.
Zippers are the best choice when you have a snug-fitting garment, like a form-fitting dress or a pair of pants. Choose an invisible zipper for side seams as well as for garments where you don't want the zipper to detract from the overall design. Metal or denim zippers are ideal for denim jeans or pants made from heavy-weight fabric, and separating zippers will work best for outerwear.
A hook and eye is usually used in conjunction with a zipper, to close the gap between the zipper pull and the edge of the garment, but can also stand on its own as a closure. Using the "eye" part of the closure isn't necessary. A hand-worked thread loop will serve the same function and will often look better.
Fabric weight is an extremely important factor when selecting snap closures. Prong-style snaps work best for lightweight fabrics. Post-style is a better option for denim and outerwear. It's a good idea to add interfacing to the area around the snaps to protect the fabric and provide durability. You can also use snap tape (or make your own with twill tape) to ensure your snaps are evenly placed.
Buttons are another closure option and work well on any fabric. Because they are available in a wide variety of colors, sizes, and styles like handmade or vintage, you can be sure to find the perfect complement to whatever you're sewing. But if you want to take it one step further, there are a few button alternatives that add even more interest.
A button and loop (instead of a buttonhole) adds a lot of detail, especially on the front of a blouse or dress. It also works well for closures at the back of the neck. You can purchase pre-made loops or use ribbon, elastic (including elastic hair ties with no metal pieces) or bias tape to make your own. It involves a bit of work, but making loops from bias tape that matches whatever you're working on creates a nice effect. You can also handwork a thread loop, which works well on delicate, lightweight fabrics such as silk and chiffon.
Frog closures will add a unique twist when used in place of buttons. They look best on vests, jackets and bags. A frog closure can be sewn on by machine (use a zig-zag stitch, tracing the shape of the frog as you sew), although it is easy enough to sew on by hand. You can purchase frog closures, or make your own out of cording or braided trim.
Toggle closures are another button alternative and a good option for outerwear. You can purchase toggles pre-assembled or make your own with toggle buttons, rope and leather.
What's your favorite type of closure to add to your finished projects?