Jewels. Sparkly barrettes and combs (you can even use an antique brooch or clip-on earring) should be pinned toward the back of the head or up near the crown, not at the sides. And restrict yourself to one glitzy chachka, unless you're actually trying to look tacky.
Garland. It's hard to pull off wearing a wreath on your head without looking like an angel in a Christmas pageant. They work only if they're small and simple, made of fresh flowers or greens, and are entirely free of baby's breath (see above reason) or jewels.
