Solo women should expect a certain amount of unwanted attention, mostly in the form of whistles and catcalls. It typically happens as they walk down the street and sometimes comes from the most unlikely sources—we saw a man dressed as Bozo the Clown turn mid–balloon animal to whistle at a woman walking by. Two or more women walking together attract much less unwanted attention, and a woman and a man walking together will get none at all (at least of this sort—street vendors are a different story).
While annoying and often unnerving, this sort of attention is almost always completely benign, and ignoring it is definitely the best response. Making eye contact or snapping a smart retort will only inspire more attention. Occasionally men will hustle alongside a woman trying to strike up a conversation—if you don’t want to engage, a brief no, gracias should make that clear.
To minimize unwanted attention, avoid wearing revealing clothing. Carrying a notebook—or creating the appearance of working—also helps.