1. Stick With What Works
When it comes to jewelry, girls are creatures of habit. The variety they apply to their wrists, necks and earlobes rarely goes beyond a double- versus single- chain earring, or a round gemstone versus a square one. This makes your task (should you choose to accept it) quite easy. All you have to do is open your eyes. If she won't leave the house without her favorite pair of gold hoop earrings, get her another pair of gold hoop earrings that have a slightly different look - hammered to provide some texture, or only a partial circle. If she frequently wears bold gemstone necklaces or bracelets, keep in mind that chunky and bold, not thin and delicate, is her style.
Along that same vein, unless she has specifically mentioned that she wants to shake things up in the gold versus sterling silver category, it is safe to assume that what she wears is what she prefers. She will probably not want to wear the new gold necklace you have given her if she has no other jewelry to match it.

2. Pick Colors Methodically
Ever overheard someone say to your girl, "that color really brings out your eyes?" Well, we're sure she has, and we doubt she'll ever forget it. There are a couple of rules to finding a color that complements eye color, hair color and skin tone. The first is that when two colors are near each other, opposites enhance, but similar tones clash. This means that if your girl has beautiful red locks, a big green necklace will make both colors much more striking, whereas a yellow or red necklace would probably dull both down to a plain, brownish mess.
The second rule is, the farther you stray from the source, the more likely that similar colors will catch and improve each other. In other words, any accessory that is meant to bring out the blue in her eyes should also be blue, since eyes are small and far away from most jewelry. If, however, you are trying to draw attention to her fabulous tan, we recommend that her earrings, necklace or bracelet be white to ensure up-close contrast.

3. Buy Something With Meaning
The truth is that we know she wouldn't ever give away that bracelet you bought her no matter how ugly it was, because she is way too sentimental for that. But wouldn't it be nice to find her something she can form both an aesthetic and an emotional attachment to? The easiest way to do that is to choose jewelry that features her birthstone (yes, someone actually chose a stone for every month of the year) or an anniversary stone, if appropriate. For a really special touch that won't take much effort on your part, do some quick web research (we recommend www.dashaboutique.com/gemstonechart.html) and print out a history of the stone, including the origin of its name and its mythical powers.
This may be the one exception to our first rule - if she has none of this gemstone in her jewelry box currently, we are sure that after she learns that it is meant to guarantee a successful marriage (Aquamarine), she will quickly make it a regular part of her wardrobe.