Step 1. Do everything in your power to learn your lady's sizes. Take a peek in the drawer for the correct bra and panty size. Rather than trying to remember what you find, write it down. Generally the bra size will have a number and a letter (for example 34C). The panties will generally be a small/medium/large or /extra large.
Step 2. Take note of the style of the lady. Is she sophisticated and chic, or sweet and girly, or perhaps a minimalist beauty. This will key you in to which direction her intimate apparel tends to go. Generally (though there are always a few exceptions) the styles tend to mesh together.
Step 3. Play it safe when it comes to color. If she generally wears lots of color, then chances are you will be safe to go in the same direction with lingerie. If she is more of a neutral girl (wearing lots of tan, gray, beiges, whites) go in that same direction. If black is her color on most evenings but she always adds something bold and bright, then go with either the black or look for something in more of a rich jewel tone.
Step 4. While you're checking on those sizes, also take note if you are finding silk or cotton. If you find more of one than the other, go with what she has more of (she probably prefers that one) If it's an equal split, then go with which ever you please. Follow these suggestions and I'm sure that you will do very well! All of the gorgeous lingerie in the photos is available for purchase through fine luxury brand Agent Provocateur. Remember gentleman, buying the lingerie in the first place is an extremely thoughtful gesture and paying attention to the details will be more appreciated than you could imagine. Good Luck!