Other people muggers avoid include dancers, gymnasts, cheerleaders, martial artists, rock climbers, professional sports players, and those who practice meditation.
In my case I'm sure it's mainly sheer size and build (well over two meters tall, a bit over two hundred kilograms). But I definitely see "muggable" and "un-muggable" (or "ready to cope with whatever happens" and "may need help in a crisis") when I look at people around me. It's definitely not *just* a matter of size and build. Somebody can be tiny and still be someone it would be clearly be not worthwhile for a mugger to mess with.
People talk about confidence, and they talk about being physically intimidating, and I'm sure those things are part of it, but it's also about environmental awareness and complete mental presence in the situation, as well as about balance and coordination.
If you want to attract muggers... so I've been told anyway by a behavioral-sciences type trying to explain why the same relatively small fraction of the population tend to get mugged over and over while others live their entire lives on the same streets without ever getting even threatened .... do any two or three of the following:
Adopt a gait most people find slightly unnatural where your left hand and left foot swing forward at the same time, followed by right hand and right foot. Wear mismatched clothing. Keep your eyes on the ground in front of your toes instead of on the people and environment around you. Alternatively keep your eyes on enormous things that tower above or in the distance from the people and environment around you. Carry something awkwardly, or have a bag or backpack heavy enough to affect your gait or balance. Act surprised at everything. Have something jammed in your pockets that breaks up the outline of your legs/hips/waist. Fiddle with your clothes as if unaccustomed to wearing that kind of clothes. Walk, not with an obvious limp, but as though one of your knees or ankles is slightly stiff. And there were several other behavioral 'markers' I forget that emerged from a study of people who got mugged more than twice in the same year.