There are several things you can do.
1. Bring a list of things you need to buy. Others will think that you're buying it for someone else or for something other than yourself. In Hollywood, for example, some people thought I was buying for a film/theatre set.
2. Talk on the phone while you're shopping. This will make it seem like you're buying for someone else who is trying to direct you on what to buy.
3. Ask one question at the end: what is the return policy on these items in case it doesn't fit her or she doesn't like them?
4. Go shopping on a specific holiday/special day, like before Christmas, Valentine's Day, etc. This will make it more like you're buying the stuff as a gift.
5. Spread out your shopping: don't buy all your stuff at one place. For example, you can buy underwear in one place, a skirt or single outfit in another, etc.