Layton's big gamble
First off, Layton has to get significant improvements in environmental legislation from the Tories. In exchange for the NDP's support, Layton will have to get the Tories to change non-action Clean Air Act to something that will give results in our lifetimes if not much sooner. This will be key to retaining any support for the party. If Jack can't get major concessions for the environment, there will be no point in supporting the Tories or making any deals for support in the future. This will look like a sell out for the NDP in any future votes where they might support the Tories.
The second thing Layton has to be careful of is what he has to give for concessions. If there is only a requirement to support the Tories where there's a confluence of interest, then there is likely to be little fall-out. If Layton has to vote for things that his core supporters can't support then there will be problems. This will alienate his supporters and send the soft vote back into the arms of the Liberals. Alienated NDP support may even leak off to the Greens which is something Layton can't afford. This will rule out any deals between the Tories and the NDP that would have the NDP support theo or neo-con pet legislation, if Jack is smart that is.
Layton also has to keep from looking too mean spirited when attacking the Liberals. The Grits aren't nearly the easy target they were even a year ago. By continually attacking the Liberals while supporting the Tories, Jack will push away his soft support and possibly some of his core support. Jack has to spend his time being very careful when attacking. He needs to attack the Tories, but not so much he can't get what he wants from them. He also needs to attack the Liberals, but not so much he looks like he's kicking someone who's down. He also has to watch what he says about the Greens. As the new upstarts, the Greens will gain much support if Layton attacks them too vigorously. In short, it's probably best if Jack tones down the attacks and pushes the NDP agenda, he'll look less mean spirited, be able to get some of that agenda through the minority house and likely hold onto some support.
If Layton can pull off these three things, he should be able to keep NDP support from bleeding away. Failure to walk the tightrope he's set up for himself will spell disaster for him and the NDP. That would be unfortunate for Canada, as much of what the NDP has to offer does resonate with Canadians, more so that what the Tories have to offer. Jack just has to be very subtle and agile to get some of it through.