It can be as simple as making a gross sound or gesture or it can be more refined using a switch. If you are going to use a switch, the person must have one reliable, consistent, voluntary movement. Switches vary with respect to force, feedback, amount of travel, amount of play, size, weight, moisture resistance and safety. There are also different types of switches. Push switches requires the person to apply force.
Pull switches require the person to pull away from the switch.
One example of a pull switch is the String Switch from Ablenet. A grasp type switch requires squeezing of the switch and a flex switch requires bending of the switch. A mercury switch is activated by position, not pressure. A pneumatic switch is a sip and puff switch. An infrared switch is one like Innocomp's BlinkSwitch.