[Mac OS X Mountain Lion] FATAL: could not create shared memory segment: Cannot allocate memory

I've just installed PostgreSQL, using brew, on Mac OS Mountain Lion.
The server doesn't start and the log file shows :

$ cat /usr/local/var/postgres/server.log
FATAL: could not create shared memory segment: Cannot allocate memory
DETAIL: Failed system call was shmget(key=5432002, size=3809280, 03600).
HINT: This error usually means that PostgreSQL's request for a shared
memory segment exceeded available memory or swap space, or exceeded your
kernel's SHMALL parameter. You can either reduce the request size or
reconfigure the kernel with larger SHMALL. To reduce the request size
(currently 3809280 bytes), reduce PostgreSQL's shared memory usage, perhaps
by reducing shared_buffers or max_connections.
The PostgreSQL documentation contains more information about shared memory
configuration.

I have been using psql before, on Lion, without this kind of prob.

What is the workaround ?

--
Yvon