The simplest way is to designate gear sets for the "lockstyle" feature. When you do this, you can use gear anywhere in any of your inventories for this purpose (gear you actually equip has to be in the main inventory or a mog wardrobe) Basically, as the person above described. But if you want to do something just temporarily, you can simply equip the items you want to show, and then do /lockstyle on, and then equip other items. Your character won't "blink" like you were probably used to and instead keep the same appearance you had before.

Because lockstyle prevents blinking, even if you aren't caring about "glamour" you should activate this feature even with your usual gear so that gear swap macros don't make you blink.