You could possibly leave your luggage at a hotel that you have status at ?

Otherwise I know there are storage lockers at berri-uqam station. these lockers are really close to where you board the bus to goto the airport too.

How much luggage are you looking to store? I am not sure of the costs.. but you can probably find out by calling the bus terminal there.

btw: berri-uqam is the starting/ending point of the 747 bus to goto the airport.