This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
The racers on the net seem to have a concensus on this (at least for
crewing on large boats). Since I wrote this originally, I got the
following words from mp, which seemed so relevant that I've put them
first: "you should add that if you want to get experience as neophyte
crew, you need to show up consistently. Most owners can put up with
you not knowing the ropes and would be willing to teach you what you
need to know as long as they know you'll be there every week."
(1) Go to local yacht clubs that have regular race series and post an
index card on the bulletin board saying that you are new to racing,
but would like a crew position. Give phone numbers where you can be
reached, and put a date on the card so that people know it's
active. (Ask the club steward about where to post the card, and
whether it's OK).
(2) Go hang out on the dock on whatever evening the local fleet races,
and ask around if anyone knows of someone who needs crew. Come dressed
for the occasion; bring a foul-weather suit if it's windy, and wear
tennis shoes or boat shoes. Have a hat. If you bring other stuff
(sweater, dry set of clothes) pack it in a small athletic bag or
knapsack. Show up an hour before race time and let various people know
you are there and available. The club steward, the launch boy/girl,
and the dockmaster are all good choices.
(3) Make it clear that you are serious-if the skipper says "can you be
there an hour before the race to help pack the 'chute?", say "Yes."
Volunteer to help out with Spring work on the boat. If you have to
miss a race on a boat on which you've been racing regularly, let the
skipper know at least 3 days in advance. Let people know that you are
willing to come out every single week to race. If not, word that you
are unreliable will get around.
(4) Listen and learn. Don't go aboard expecting to tell everyone
everything you know. If it turns out that you know more than they do,
keep quiet about it. Your quiet competence will eventually show
through.
 
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