If it was just the two of us at her house, I wouldn't say anything.
If she had other guests or if we were going to go somewhere together and I didn't know how to broach the subject without sounding critical, I might somehow contrive to get her in front of a mirror. If we were on our way out to a restaurant or bar, I might take her aside and say I wanted to check the back of my outfit and ask her if she had a full-length mirror I could use. Once there, I might say something to get her to stand in front of it too, like "Hey, come tell me what you think...I loved this skirt at the store, but now that I've been wearing it for a bit it's been bothering me. Is it just me or does it look way too tight/really lopsided/etc." Then comment on the lovely color of her own shirt or something while we were still looking in the mirror. That way, she would get a chance to see her whole outfit and make her own judgement about it without me saying anything.
P.S. Love your rubber band trick JenJay!