Reverse SSH tunelling

Hi All,

        I need some advice .. I have a situation where about fifty servers will
be located in fifty sites that cannot allow services to be hosted. These
servers will be in private network space behind firewalls. I can use
them to 'scp' files out to a common home base server, but sometimes I
need to access a command line console on these servers. I am thinking of
having a hourly cron job ssh out to my home base server and leaving that
tunnel open  so that I can access that console, but am looking for the
specific way of doing this. Security os pf the utmost concern, so I need
some sort of encrypted tunnel, hence the thought of ssh, but I don't
know how to do this 'reverse' tunnel... I was also thinking of a 'free
swan' vpn tunnel .. 


Thanks

Raist