Help setting up warm standby replication

Hi,
after having read the documentation and the few posts in this list I
tried to set up
a warm standby replication between two instances of postgres running on
my pc
in 2 different base directories and 2 different ports.
(The second one was a cp -a of the first one after having created a
backup point) .
I don't need a realtime update: even few minutes of lag are good enough
for my needs.

To begin with I followed the example of the docs: in the recovery.conf
file of the
slave instance I set

restore_command = 'cp -av /var/lib/pgsql/data/pg_xlog/%f %p'

but at restart I got these messages in the logs (some translation from
italian to english by me) :
LOG: system shutdown at 2007-04-13 13:02:51 CEST
LOG: starting archive recovery
LOG: restore_command = "cp -av /var/lib/pgsql/data/pg_xlog/%f %p"
cp: impossibile fare stat di
`/var/lib/pgsql/data/pg_xlog/00000001.history': No such file or directory
`/var/lib/pgsql/data/pg_xlog/000000010000000000000007' ->
`pg_xlog/RECOVERYXLOG'
LOG: restored file "000000010000000000000007" from archive
LOG: invalid record length at 0/7000218
LOG: invalid primary record checkpoint
`/var/lib/pgsql/data/pg_xlog/000000010000000000000007' ->
`pg_xlog/RECOVERYXLOG'
LOG: restored file "000000010000000000000007" from archive
LOG: invalid record length at 0/70001D0
LOG: invalid primary record checkpoint
PANIC: impossible locating a valid record checkpoint
LOG: startup process (PID 20250) was terminated by signal 6
LOG: aborting startup due to startup process failure

same thing if I empty the pg_xlog/ dir (except archive_status/)

I hope someone can explain what I did wrong.

Another thing I didn't understand is the continous replication mechanism:
will the slave server periodically call the recovery_command or do I
have to setup a cronjob to instruct it to search updates?

Sorry if these questions are stupid :) and thanks in advance.

Nico