Arguably the most time and cost-efficient way of getting from Toronto to Montreal and back is by train. Canada's national rail line, Via Rail, runs several trains a day between Canada's two most significant urban centres, including multi-stop milk runs and one-stop express routes.
The train is significantly cheaper than flying, and when you factor in time spent getting to and from the two cities' suburban airports and other typical air-travel vagaries, Via's service is almost as fast. Tickets can be purchased at either city's main train station; Montreal's is at University Street and Boulevard Rene Levesque, and Toronto's is at Front and York Streets.
The only significant downside to travelling by train is having to spend four and a half hours listening to garrulous residents of the two cities discussing the differences between them, which are based on Montreal's essential 'European-ness' and Toronto's basic 'American-ness'. This is charming the first time, bearable the second time, and desperately wearying every time thereafter.