Fresca, I loved your experiences :-)
In my country, there are no separate "male" and "female" fitting rooms so it's far easier for us to try things out (couples are fond of entering them together, so they can comment on each other's choices!). Perhaps the only snag for the more shy types is that most stores will carefully count how many pieces of clothing you take with you inside the fitting room, so the shop assistant will find it amusing if a guy takes a lot of bras an skirts into the fitting room. Amusing, yes, but they will not say anything — if you're a customer, you're a customer.
In fact, while I cannot claim to have a huge shopping experience (I'm too poor for that!), on all occasions I've shopped for clothes or accessories, I was always well treated. Originally I tended to pick the winter holidays season for shopping, because it would be less obvious claiming to be buying for a female friend. But people have birthdays all around the year, so it's always a good excuse as well. Sometimes, however, I skip the explanations altogether.
There was just one case so far where I had no possible excuses: trying out a pair of shoes. I wear huge sizes, and people usually don't shop for shoes for their other-gender relatives anyway, so it was obvious I was choosing shoes for me — specially because I had to visit the "large sizes" aisle.
So after browsing for a while on one shop that carries large sizes, a shopping assistant came over to me (I wonder if they drew lots to see who would attend to the "weirdo"). I was in drab mode — actually, in male business clothing, to be more precise

The shopping assistant was a bit shy, but it was clear that she was used to seeing males around the large size aisle, and she offered her help. While I'm not still completely 100% confident about shopping for female clothes and accessories, I'm always nice, polite, and smiling — the worst that they can do, after all, is kick me out of the shop! — so that put her to ease. I asked a few advice on some of the models and asked her if I could try them out for fitting; she said it was fine. I did it by the aisle, standing up — slipping out of my male shoes and testing one or two female ones for size and fit. I'm actually glad I did so, because, as usual, the size numbers on the shoes were not exactly correct (they hardly ever are, in my country). To this day, and except for some boots I bought on eBay, these shoes are the ones fitting me best

So that was it, nothing very special, lol
Where I got more eyebrow raising was not in a shop, but by getting some of my items sewed back and/or slightly changed. There are lots of small shops doing that kind of work. I picked one in a low-profile shopping mall and brought them some things to re-fit and sew back again. They showed some surprise but did their job well — I sort of told them I was part of a group of amateur theatricals (there is a theatre nearby) and that I had more "props" to fix. Later they got used to fixing "strange" things like corsets and padding items and so forth. I think they found it rather amusing!
For several reasons, I was unable to use them again and had to find another shop, near my place. Here I got into some difficulties. I have a slightly oversized dark olive dress, in a straight cut, and I wished to give it a waist, because, obviously, straight cuts don't fit me well (but I had no idea of that when first buying that dress!). So I took very precise measurements and told them exactly what I wanted. Here I hit a snag: they questioned my measurements and wondered about them (I'm rather curvy with the padding!). So they didn't want to do the job unless they could do a fit! I had told them it was a surprise present for one female relative, so I couldn't bring her to the sewing shop. They were very reluctant to accept the job, but, at the end, when I was sort of giving up on them, they sort of made me promise that I would not complain if the dress got ruined due to lack of proper measurements. I naturally accepted their conditions! And, to this day, this is one of the dresses that fits me best (I'm wearing it today as a treat).