how to choose secure PPTP password?

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http://serverfault.com – I'm running a PPTP server on a ubuntu VPS, which has around 20 users . currently we use passwords like this "bi9_Knh78A#" . is this considered safe ? What's the best method to choose passwords ? How secure is PPTP VPN when passwords are very strong ? any tip to increase security ? (HowTos)