The first key thing is the project itself. If the project is not interesting, there is no way there would be a conversation around it no matter how much effort is put into it and even with hiring of people to bump the thread, its frown at which might lead to red on the forum.
After that is settled, the next is presentation, a lot of those topic that you see as "very hot", check the opening message its very detailed and interesting. Its based on that information that propels discussions that run into several pages.
The most important that keeps the thread and discussion going in the community manager and the developer who are well grounded in the project and able to offer timely response to questions raised. With other efforts such as signatures with links to the thread, the objective will be met. All this put together is not a guarantee but a step in the right direction.