Synchronization Master -> Slave

I've serious problems with slow link between continents, and twice a week I have to manually reestablish the slave, by running the following script:

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psql -c "SELECT pg_start_backup('standby_sync', true)"
rsync --progress -a ${PGDATA}/* root@server2.mycompany.com:/opt/pgsql/slave1/ --exclude postmaster.pid --exclude postgresql.conf --exclude pg_hba.conf --exclude pg_log --exclude pg_xlog
psql -c "SELECT pg_stop_backup()"

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Point is not to analyze the connection issues between the continents, but if there is a way better to synchronize these servers, and avoid to transport 8Gb of data between them when syncronizing.
The rsync above can be optimized? Both servers are CentOS 5 with OpenVPN established between them (this is one of the problems: the VPN keep falling several times a day, and then finally, the sender and receiver process just vanish in the air - and I had no time to investigate the vanishment).

Thanks in advance for your help,