Wider straps make her shoulders and arms look thinner.  They also support a C, D, DD or E cup.  Thin straps wear out too soon and don't keep things up or  in place.  This is very annoying!

Spaghetti straps look sexy but are annoying to most women because they fall off her narrow  shoulders.  This means the garment is likely to remain in a drawer.

Over a C cup always choose underwire garments (bras, corsets, bustier and slips/camisoles when possible) unless she's indicated she dislikes this

Structured or tailored garments (corsets, bustier and A line slips)  look better on most women, especially if she is voluptuous.

Red is the one color that looks great on any woman who wears it.  Black or White tend to make the wearer look pale and pasty.

Unless she's model thin, go for coverage over skimpy. Leave something to the imagination.  Lace, satin or silk conceal. If she's comfortable in it, she'll wear it.

If you can find fancy silk lingerie/garments, these are the best.  Silk breathes and feels soft.  It is also very strong and doesn't stretch like cotton. Most lingerie is made out of satin or lycra,  which are synthetic and do not breath.  If the garment is synthetic, look for cotton linings, especially in the crotch.

If you're not sure of her size, opt for your smaller guess.  She will return it but will appreciate the fact you think she is smaller than she is.

Women's lingerie goes by bra size.  You'll need to sneak a peak at hers to get the right size.

Bottoms go by S (2,3,4), M (5,6), L (7,8), XL (9,10) or sizes 1-10. The larger she is, the more coverage she'll want to feel sexy.