I'd like me-sos@my.host.name to be forwarded to a bunch of people. 
Answer:
Put a list of addresses into ~me/.qmail-sos, one per line.  Then
incoming mail for me-sos will be forwarded to each of those addresses.
You should also touch ~me/.qmail-sos-owner so that bounces come
back to you rather than the original sender. If you want subscriptions to be
handled automatically, put
| qlist2 sos my.host.name
into ~me/.qmail-sos-request. Anyone who wants to subscribe can
simply send a message to me-sos-request@my.host.name. 
Alternative: ezmlm (
http://pobox.com/~djb/ezmlm.html) is a modern mailing list
manager, supporting automatic subscriptions, confirmations, archives, fully
automatic bounce handling (including warnings to subscribers saying which
messages they've missed), and more. 
I tried setting up ~alias/.qmail-P.D.Q.Bach, but it doesn't
do anything.
Answer:
Use .qmail-p:d:q:bach. Dots are converted to colons, and uppercase is
converted to lowercase. 
.forward files with qmail? 
Install the dot-forward package, 
http://pobox.com/~djb/dot-forward.html. 
/etc/aliases with qmail?
Install the fastforward package, 
http://pobox.com/~djb/fastforward.html. 
To make majordomo 1.94.* work with qmail under fastforward, insert 
system("newinclude","$listdir/$clean_list");
before the lclose(LIST) line in do_subscribe and do_unsubscribe
in majordomo. 
See 
ftp://ftp.eyrie.org/pub/software/majordomo/mjqmail and
http://www.qmail.org for
other methods of using majordomo with qmail. majordomo 2.0 is expected to
support qmail directly. 
If ~joe suddenly disappears, I'd like mail for joe to be deferred. 
Answer:
Build a qmail-users database, so that qmail no longer checks home directories and the password database. This takes three steps.
/var/qmail/users/passwd. 
qmail-pw2u -h < /var/qmail/users/passwd > /var/qmail/users/assign
Here -h means that every user must have a home directory; if you happen to
run qmail-pw2u during an NFS outage, it will print an error message and
stop.  
qmail-newu
Make sure to rebuild the database whenever you change your user list.
I set up ~alias/.qmail-www, but qmail is looking at
~www/.qmail instead. 
Answer:
If you run
chown root ~www
then qmail will no longer consider www to be a user; see
/var/qmail/man/cat8/qmail-getpw.0. For more precise control over
address assignments, see /var/qmail/man/cat5/qmail-users.0. 
procmail with qmail?
Put
| preline procmail
into ~/.qmail. They will need to use a full path for
procmail unless procmail is in the system's startup PATH. 
If you are moving from sendmail, and users have procmail in their
.forward files: procmail will continue to work from .forward
files if you have installed dot-forward, see question 
How do I use sendmail's .forward files with qmail?. 
If you are moving from sendmail, and sendmail was using procmail
instead of binmail: Copy /var/qmail/boot/proc or
/var/qmail/boot/proc+df to /var/qmail/rc. 
elm's filter with qmail?
Put
| preline filter
into ~/.qmail.
They will need to use a full path for filter unless filter is in
the system's startup PATH.
I'd like to set up a LUSER_RELAY pointing at bigbang.af.mil. 
Answer:
Put
| forward "$LOCAL"@bigbang.af.mil
into ~alias/.qmail-default.