Shoes - Metallic heels, including sandals, are always right for formal occasions, but can also be worn for daytime weddings, and many metallic flats go with almost any outfit. Black patent leather is even more versatile, and is dressy enough for the most formal attire.
Outerwear - Please invest in a presentable, dressy coat to have for formal occasions, or brave the cold without one. Black always makes a coat or jacket more formal, so consider buying something black, fitted and long enough to cover a dress. It will go with more casual outfits as well.
Handbag - A small clutch or shoulder purse (about 5 x 7 inches) is much better for evening than your everyday large leather bag. It can be sparkly, metallic or more simple black or a colored fabric.
Jewelry - You don't need expensive fine jewelry. Larger costume jewelry pieces, such as those found at J. Crew, make a statement and highlight your outfit. A little black dress with big, bold, jewel-toned jewelry looks striking and makes a strong statement. Use both necklaces and bracelets, and don't match everything. They should coordinate, but not be from a matched set. You'll find these can be worn with a lot of your more casual outfits as well.
You don't have to spend a lot on any of these accessories. I like to keep my special occasions in mind all year when I'm shopping and if I see a beautiful piece of jewelry on sale, or a fun clutch bag, I'll go ahead and get it, then work it into my outfit when the right time comes. Meanwhile, I try to use sparkly and metallic pieces in my regular wardrobe.
Get more tips on dressing for all occasions and for your everyday life in my newly updated book, Six-Word Lessons to Look Your Best.