Another useful method I find is to get a Temporary Suspension for the time you are gone as it completely turns off the services of your device.
This lowers the monthly rate of your plan while you have the suspension applied, so you actually will pay less for the time you are gone and not using the plan services (therefore saving money), and you don't have to worry about the device accidentally being connected to the network briefly when you turn the phone on, or accidentally turning off the Airplane Mode by hitting the wrong button, or accidentally misplacing/breaking the SIM card, etc.
In most phones, airplane mode is shutting down ALL wireless signals. That includes wireless, wifi and bluetooth. Maybe the odd phones would leave the wifi running and only shut down the wireless signal. If you want to avoid accidental roaming charges but still want to use your phone's other functions, the safest bet is to use the phone without the sim card. You will get the sim card missing error message in most cases.
What I do when "roaming" (I don't have a data plan) is do all my calling via VOIP (e.g., Skype's option to call telephones). Recently I needed to be on an hour-long conference call while on a train in the US without wifi, so I just purchased the 50 mbytes for $7.99 now available, and it was more than enough for the full hour and then some.