Casual, vaguely post-hippie; T-shirts, jeans, running shoes, Birkenstocks (or bare feet). Long hair, beards, and moustaches are common. High incidence of tie-dye and intellectual or humorous `slogan' T-shirts. Until the mid-1990s such T-shirts were seldom computer-related, as that would have been too obvious - but the hacker culture has since developed its own icons, and J. Random Hacker now often wears a Linux penguin or BSD daemon or a DeCSS protest shirt.
A substantial minority prefers `outdoorsy' clothing -- hiking boots ("in case a mountain should suddenly spring up in the machine room", as one famous parody put it), khakis, lumberjack or chamois shirts, and the like.
After about 1995 hacker dress styles assimilated some influence from punk, gothic, and rave subcultures. This was relatively mild and has manifested mostly as a tendency to wear a lot of black, especially when `dressed up' to the limit of formality. Other markers of those subcultures such as piercings, chains, and died hair remain relatively uncommon.
Very few hackers actually fit the "National Lampoon" Nerd stereotype, though it lingers on at MIT and may have been more common before 1975. At least since the late Seventies backpacks have been more common than briefcases, and the hacker `look' has been more whole-earth than whole-polyester.
Hackers dress for comfort, function, and minimal maintenance hassles rather than for appearance (some, perhaps unfortunately, take this to extremes and neglect personal hygiene). They have a very low tolerance of suits and other `business' attire; in fact, it is not uncommon for hackers to quit a job rather than conform to a dress code. When they are somehow backed into conforming to a dress code, they will find ways to subvert it, for example by wearing absurd novelty ties.
Female hackers almost never wear visible makeup, and many use none at all.