You can't walk into a shop and guess at your partner's size. A woman's bra is almost like a second skin, so if you choose the wrong fit she'll never wear it. If you don't know her measurements and would like to keep your lingerie shopping a surprise, all it takes to is a little Bond-style espionage to find out. Simply take a look at the laundry when the lucky lady isn't around and make a note of her labels.

With bras, there are two sizes to keep track of. The first is the cup size, which can range from A to K. The second is a number, which relates to the measurement around the body and rib cage. It's important to note that the two numbers are linked: for example, a 32D will not have the same size cup as a 38D because the letter relates to the volume of the bust itself and the size of the cup in relation to the band. You won't be able to buy a 32D bra for a woman who wears 38D, even if you get it altered. Bra sizes can vary across lingerie brands, so it's a good idea to check a couple of bras that your partner wears most often – these are most likely to be the ones that fit her best.

If you're really struggling or don't feel comfortable peeking into her wardrobe, you could try and find a negligee or camisole instead. You don't need to be as exact with the sizing for these flowing garments.

There are lots of different shapes and types of bras out there. Here are the most popular:

• Balcony bras – Lift the breasts to enhance the cleavage with straps that support from the bottom of the cup. Super seductive.
• Plunge bras – Create deeper cleavage by plunging down, supporting from the centre and pushing the breasts together.
• Full-cup bras – Offer the best support and shape by covering more of the breast. Usually worn by women with a larger cup size.
• T-shirt bras – Best for wearing under a T-shirt. Made from smooth material and normally seamless, they are less likely to be seen through the fabric.

Women's knickers also come in a tremendous variety of styles. Think about the underwear she normally wears and see if you can match it to the lingerie types below for an idea of what she likes. There are lots of different shapes and types of bras out there. Here are the most popular:

• Full brief knickers – Offer the most coverage with a high waist.
• Hipster knickers – Sit lower on the hips and usually cut away at the back.
• Thongs – Well-known for their high leg which exposes the bottom.
• French knickers – Usually made of lace with a high waist that sits half way across the buttock.

If you want her to be thrilled with your selection (so you'll be allowed to buy lingerie again) keep these final pointers in mind:

• It pays to pick a colour that you know she'll love. Be observant and get to know her style, her favourite outfits and the colours she looks best in.
• Use colour language to say something with your gift. Black is generally thought to be sultry and classic, white makes a wonderful wedding present, while soft pastel tones exude femininity and romance.
• Pick a lingerie set to make sure the underwear matches perfectly.
• Remember that this gift is for both of you. Keep her style preferences and comfort in mind, but make sure it's also something you'd like to see her wear. You'll both enjoy the gift more knowing that your partner is as excited about it as you are.
• If you're unsure of her knicker size and don't want to intrude on her underwear drawer, check her dress size. The two are usually very similar.
• Make a full ensemble by adding a waspie or suspender. Lingerie accessories are always exciting.
• Surprise her! Leave a gift waiting for her when she arrives home.