Well first of all, do not buy her cheap underwear from Walmart. Go all out and visit Victoria's Secret or a nice department store.
Step 1 is to sneak into her drawer to find out what size she is.
Step 2 go to the store and scope out the nice underwear, not the cotton granny panties! Buy her some sexy ones that are made of silk or lace.
Step 3 pick out a few pairs and explain to the store clerk that you are buying these for your wife. Ask her if they are a popular item or ask her to help you pick out something special your wife may like.
Step 4 wrap them up in a nice little box and surprise your wife. She will be so happy and surprised!
Good luck :)