The lion and the unicorn
Were fighting for the crown
The lion beat the unicorn
All around the town.
Some gave them white bread,
And some gave them brown;
Some gave them plum cake
and drummed them out of town
Not so fast. At this stage, neither alternative party wants the public to think that a vote for them is a vote to keep the Conservatives in. Simple principle: anyone who wants the Conservatives in is voting for the Conservatives.
But what actually happens if the Conservatives win more seats than anyone else?
Harper would have the right to be appointed Prime Minister by the Governor-General. It would be a public scandal if he were not. Then it is up to the other parties to vote him down. Let's say they vote him down. Then, if the election has been quite recent, the Governor-General would properly go to the leader of one of the other parties to see if they can form a government. He might instead violate tradition and call an election, but if he did, the waste of money and apparent lust for power would be a big issue, and the Conservatives would not be the ones blamed.
Now, while it may not be in the interest of either the Liberals or the NDP to see the Conservatives stay in power, it is much less in their interest to see the other alternative party given a chance at government. Since they compete for more or less the same ideological constituency, this would be a crippling blow, perhaps a death blow. It tags their rivals as the apparent alternative.
Therefore, it becomes in their interest not to force it to this point. Given that the Conservatives do not bait them with something they cannot support without looking duplicitous, the party in third place when the election is over is pretty sure to vote with the government for the first six months at least—until it would look as if the Governor General would have to call an election if the government fell.
They could insist, of course, that they are not "propping up" the government at all, but merely supporting this or that specific measure which they feel is in the best interests of Canadians.