I work retail, and now I don't do retail MSes, so here's my perspective as an associate. We do occasionally get returns within a short time. It doesn't really matter what your reason is! Associates don't need a back story. Is it defective? Please let me know so I don't put it back on the floor. Would you like another color or size? If so, please let me offer a replacement to satisfy you, which is my job, which you may certainly decline. Otherwise, please just be polite; I know life happens, and I'll just process the return cheerfully. The observations of associates where I work is this: if the customer offers a story more than one sentence long about a return, we smell bull, and we check the merchandise, tags, and receipt very, very, very carefully, which takes longer. The bigger the guilt, the bigger the story, they say.
So my advice for MSers is this: be natural and be polite. You're returning an item, for which you are entitled refund of your hard-earned money (as long as that's the store policy). You don't have to provide any more explanation than if it is damaged, or if we can satisfy you with a replacement. "Hello. I need to return this dress. I have the receipt." That works for me. You don't need to be "sorry," because it's your money, and it's our policy to take returns. If you're nervous, we'll be suspicious of you. If you're polite, we'll be happy to see you come back, and it won't even register that you returned something.
(Now, if I personally helped you for two hours, and we built a strong rapport, you might mention a reason because of that. "I didn't get that new job, after all. But I really appreciate your help earlier, and I'll be back." That would be cordial, but certainly not required, especially on a MS. I've taken returns on big dollar purchases for customers that took hours of my time; they offered no stories, and that was just fine.)
Usually my coworkers remark about the process or the day's sales totals, not the customer herself. Me, I tend to just take returns as a part of doing business. It's an easy thing to do, and there's no need to fret.