1. If it's too small, we feel like a whale in a puppy harness. If it's too big, we're convinced you think we're huge. Before heading to the store, go through your gal's underwear drawer, check her bra and panty tags, and take notes on the digits.

2. Personal style is as important for underwear as it is outerwear. Just because you're into tassels doesn't mean she is, so pay attention to what she usually wears and what you could imagine her liking. Pushing the envelope slightly will be seen as adventurous; pushing it too far, plain creepy.

3. If you're still feeling completely clueless (or if she has a padlock on her panty drawer), let her do the buying. A week before Valentine's Day, present her with a gift certificate to a classy shop—French Dressing, Lingerie Studio, and La Perla are good local options. And don't be stingy with the dollar amount. A hundred bucks is decent. Three hundred or more, and you're a god.