1. Check the dirty laundry. While you woman is in the shower, check her bra and panty size. Nothing ruins a sexy gift of lingerie quicker than buying her something two sizes too big. No matter how good she looks, she's self concious about her figure.
2. You are not shopping for Heidi Klum. Be realistic about your selection of lingerie. Buy something appropriate to your woman's body type. Does she have a baby belly, and an awesome rack? Shop a baby doll which draws the eyes to the cleavage, and covers the belly. That corset may look hot on some 90lb boob jobbed model, but your woman will hate it. She may actually want to draw a breath at some point in the evening.
3. Don't be shy. When you are at the lingerie store, get one of the ladies to help you. Now don't be a creep about this, but try to pick out a sales lady with your lady's body type. Give her your woman's measurments early, and she can lead you to something appropriate.
And again, don't be a creep. Keep your eyes on the task.
4. KISS Keep It Simple, Stupid. Think of lingerie as gift wrapping. You want easy access. You do not want to be fighting with a half dozen doubleback hooks, behind her back, while in the throes of passion. A drawstring at the cleavage is perfect. Fun and easy to reveal the prize inside.
5. Complete the package. So you've got some lingerie she will look hot in. Does it need stockings? Again, consult with the sales lady. Buy her favorite wine, or choclate covered strawberries.