Here at Cranfield University we use Raspberry Pi computers for a number of applications, such as monitoring environmental sensors and processing data. The devices get moved around the campus and need to work across multiple WiFi networks with ease ad without the need for reconfiguration. Here is how this is done (make backups of the files before editing!).

First, edit file /etc/wpa_supplicant/wpa_supplicant.conf

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
	ssid="SSID_1"
	psk="PASSWORD_1"
}
network={
        ssid="SSID_2"
        psk="PASSWORD_2"
}

Second, edit file /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback

# The primary wired network interface
iface eth0 inet dhcp

# The wireless network interface
allow-hotplug wlan0
iface wlan0 inet manual
wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf

# Default
iface default inet dhcp

Reboot the Pi after the edits. This approach uses DHCP for connecting to the network.