I do not have to wear a certain outfit, I am a lifestyle Domme and my pet serves me while I wear anything from a gown to pajamas. However, if I want to heighten the mood, or am going to an event, I would usually have a few specifics. I usually style my hair so it is down, as it is very long, but I'm prepared to put it up when I scene, so it can't get caught in a wayward flogger or something. I like corsets and stockings, I also prefer heels. I usually do my makeup, particularly eye, with an intense tone. My go-to item to carry around is a riding crop. Most of my floggers are too bulky to carry with me, but a crop is lightweight and effective. It can also help make for an intimidating visual effect.
Overall, it is specific to you. What makes you feel Domme? Because that is where you need to focus your energy. If a particular toy or outfit empowers you, then it will come across to the submissive, and everyone else around you. So try things on, or look around online, until something speaks to you. I always know when I've found the perfect accessory, because I can't live without it!