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 (only rarely computer related; that
would be too obvious).
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.
Very few 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.
Female hackers almost never wear visible makeup, and many use
none at all.