When can I clean up archived WAL files?

I need some help understanding the how archived WAL files are used and when I can clean them up.

Currently we have a master database replicating to a slave, but every so often our master database server runs out of disk space because the files archived using archive_command build up and we have to manually remove them. I feel like we're missing something if we have to manually remove old archived WAL files because there are too many that the file system runs out of disk space...

Having said that, I'm not sure I understand the replication process enough to start deleting files and be confident that I'm not screwing anything up. Does our slave use the archived files created by archive_command or does it rely only on the files in $PG_DATA/pg_xlog directory to maintain a consistent state with the master?

If that's the case, and we're doing a pg_dump of our master every six hours, could I update our backup script to also clear out all archive files that exist at the start of the pg_dump (once the pg_dump is complete of course)? from the docs:

Note: pg_dump and pg_dumpall do not produce file-system-level backups and cannot be used as part of a continuous-archiving solution. Such dumps are logical and do not contain enough information to be used by WAL replay.

I just want to make sure that however we clean up the archived files, we still have what we need for replication to continue as well as be able to perform a PITR in since the last base backup.

Our archive command is cp %p /var/lib/postgresql/9.2/archive/%f and here is our recovery.conf file from the slave:

standby_mode = 'on'
primary_conninfo = 'host=<ip-address> port=<port> user=rep password=<password> sslmode=require'
trigger_file = '/tmp/postgresql.trigger.5432'