Many cafes limit wifi usage by keeping track of your MAC address. To get around this limitation, simply spoof your MAC address every time your session expires.

How to be a cafe terrorist (Mac version)

# create the following symlink if the airport command in the next step cannot be found
sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/sbin/airport

# disconnect your machine from the wireless network without turning off airport
sudo airport -z

# spoof a mac address on your wireless interface (random hex values work fine)
sudo ifconfig en1 ether 00:1f:5b:ce:0d:e4

# reconnect!