Avoid bringing any sources of ignition (cigarette lighters, electrical tools or appliances) into the area where the smell is detectable. Even the smallest electro-static spark could ignite the gas.
Wet the gas canister before touching it.
Move leaking canisters into the open air to a place where the gas can flow out, e.g. onto open ground.
Try stemming the flow of gas by shutting the relevant valve (e.g. the outlet valve on a canister, the main gas tap at the point of entry to a building, etc.).
Try to locate the leak by spraying soapy water onto possible leakage points. This can be done using a simple plant sprayer. Soap bubbles will form around leaks.