It is good etiquette to let the store clerk know that you would like to try some clothes on before taking clothes to the fitting room.
Refrain from taking clothes from their displays and entering the fitting room without being guided to it by the staff.
It is also considered good manners to wipe off the sweat from your underarms and back before fitting clothes.
For women, take care not to get lipstick or foundation on the clothes.
(Many stores have disposable face covers for exactly this purpose.)
Bring only about two to three pieces of clothing to the fitting room per fitting.
Treat products with care and respect. Do not treat them roughly or leave them scattered all over the fitting room.
If there are plenty of people at the cash register, form a single line and allow the person at the head of the line to use the first available cash register.
Payment of the 8% sales tax is mandatory unless you are shopping at a duty-free store.
Most of the time, prices on display are not inclusive of sales tax.
Plenty of stores in Japan still do not accept credit cards, compared to their counterparts overseas.
Especially small-time mom and pop stores are likely unable to accept credit card payments.
Electronic money have become more widely-used in Japan in recent years, but the number of payment processors that are available locally and overseas is limited.
If you intend to use an electronic money for shopping during your visit to Japan, choose one that is usable in Japan as well.