Certain halachic issues are informed or even determined by the social standards of the (non-Jewish) society in which one lives. Examples include what falls under the prohibition of cross-dressing (see Rambam Avodas Kochavim 12:10, Shulchan Aruch Yoreh Deah 182) and standards of modesty that extend beyond the letter of the law (e.g. where it is socially unacceptable for a woman to go out without a veil, it might be halachically required for a Jewish woman to adhere to this standard of modesty beyond the basic objective halachic requirements - see http://hebrewbooks.org/rambam.aspx?sefer=4&hilchos=22&perek=13&halocha=11&hilite=).
My question is: what is considered "common practice" for the purposes of such halachos? How common does the practice have to be to permit it? Does it have to be done by the majority of people, enough people that you won't get weird looks if you do it, a minority of people that will nevertheless not be viewed as, for example, cross-dreessers for wearing a certain article of clothing, etc.?