Networking Setup

On master:

On the master, set ROS_IP to its IP address (since other computers probably can't resolve its name on the network) and run roscore:

export ROS_IP=<ip address>
roscore

roscore will print out ROS_MASTER_URI, which needs to be set on all other machines.

On slave:

Using the ROS_MASTER_URI displayed on master:

export ROS_MASTER_URI=http://<ip address>:11311/

You will probably need to set ROS_IP on the other machines with their own IP addresses also, if they are publishing anything. In your ros_workspace/setup.bash, you can read the computer's IP address automatically every time you open a new shell:
export ROS_IP=`ifconfig wlan0 | grep 'inet addr:' | sed -r 's/^.*inet addr:(\S*).*$/\1/'`

Errata

For more info see:
http://www.ros.org/wiki/ROS/Tutorials/MultipleMachines
http://www.ros.org/wiki/ROS/NetworkSetup