Stuck trying to backup large database - best practice?

Hi,

We have a postgres 9.3.x box, with 1.3TB free space, and our database of
around 1.8TB. Unfortunately, we're struggling to back it up.

When we try a compressed backup with the following command:

pg_basebackup -D "$BACKUP_PATH/$TIMESTAMP" -Ft -Z9 -P -U "$DBUSER" -w

we get error:

pg_basebackup: could not get transaction log end position from server:
ERROR: requested WAL segment 0000000400002B9F000000B4 has already been
removed

This attempted backup reached 430GB before failing.

We were advised on IRC to try -Xs, but that only works with a plain
(uncompressed) backup, and as you'll note from above, we don't have enough
disk space for this.

Is there anything else we can do apart from get a bigger disk (not trivial
at the moment)? Any best practice?

I suspect that setting up WAL archiving and / or playing with the
wal_keep_segments setting might help, but as you can probably gather, I'd
like to be sure that I'm doing something sane before I dive in.

Happy to give more detail if required.

Antony