1. When she asks you opinion, try and find 1 thing to say. "I like the color." "It's a little too expensive here." "Is it the right size?" Just one little thing is all she needs to hear.
2. Try and go wth her attitude. If she obviously liked something, then like it also. Same goes when she doesn't like something.
3. If you truly have no opinion, just say "I love it!"