Firstly, it should be appropriate to your relationship, otherwise it could come across as sleazy or too eager, especially if it’s a new relationship. Lingerie is an intimate gift to be given, and romantic, so don’t give it to her in front of her parents or your mates!
Now for the fun part, and if done discreetly can be quick and painless, as long as you have done your homework. By that I mean observe what drawer she goes to for her bras, DON’T ASK. I’m presuming you already know her favourite colour. If not, this can be asked, but don’t just come out and ask her “what colour are your bras and undies”!
Don’t panic, I know you don’t know the size or style of bra she wears, but we have that covered. However it does require some snooping on your behalf, (I knew I would have your full attention now).
The art of snooping is to be quick, know what you’re looking for and whatever you do, don’t get caught going through her Lingerie drawer, because this will look really bad from her point of view. Explaining what you are doing, will then blow your surprise! Choose a time when she is (still at work, out with friends or even in the shower). Once you’re in the drawer, find a few bras, note their colour, look for a legible tag, does it have underwire along the cups and remember the sizes (yes there may be a few different sizes). Even better, lay them out on the bed and take a photo of several of the tags. This way you can go to the Lingerie shop prepared and not stand there awkwardly not knowing where to start. This however is not perfect, but a lot better than guessing her size.
If you have seen that she wears pastel colours in a size 32A, please don’t buy a bright red 32 or 34 in a DD cup. She won’t appreciate the gift and believe me she will probably take it the wrong way. Also, don’t be tempted to buy something you would like to see her in, e.g. crutch less undies.
If you’re running out of time, a gift voucher from a bra boutique or department store will still give you many brownie points!
She is going to love all the time and effort, you have put into her gift. Keep your receipt, just in case and well done!