I understand this is not the "fix" you were hoping to receive, but I've always found that the best way to deal with this situation is to avoid it altogether. A team that works tons of overtime for long months is not only going to run into "burn out" issues when it comes to the next project, but they are going to be much less effective at their jobs during that overtime.
Rework touches on this topic in two separate sections: 'Go to sleep' and 'Send people home at 5'. They worded it more eloquently than I had been able to prior to reading those passages, so I'll explain via paraphrasing the book:
- As people work on a project and get tired or burnt out, they become stubborn and will continue to try to push through whatever path they've chosen in a constant effort to reach the 'finish line' in the distance. The result is they don't make smart choices about when to change their tactics/processes/solutions.
- Creativity is diminished. Reword posits that more productive people simply use their creativity to find ways to do things in a shorter amount of time rather than beating the problem over the head with tons of man hours. The more you slam people against a problem, the less creative they become.
- Morale suffers. This one is fairly obvious, but should be considered to be a problem just beyond employee satisfaction since low morale means loss of motivation to do things the 'right way' or to tackle larger issues that they would normally jump into head-first.
- People become more irritable and communication and overall team efficiency can suffer as a result.
- You get used to doing things in inefficient ways. Rather than working smarter to get the problem done, you simply throw more hours at the problem.
In the long run, the easiest way to bring their enthusiasm and engagement back is to inform the team that the last project helped you to realize that the team needed to strive toward working smarter, not harder. It can be hard to justify this to higher-ups sometimes, but if you can analyse previous projects where you hit "crunch time" and point out places where you had to go back and redo parts (thus wasting time) or that the number of issues/defects was higher than normal (thus wasting time), you begin to build evidence to the fact that you're hurting both the projects and your team more than you are helping anyone by working 50-80 hour weeks for several months.
The alternative, I'm afraid, is usually to overwork them to the point where they start looking for other jobs. Some people are more tolerant than others, but if that kind of project is your 'norm' then I would expect to see a high turnover rate. Typically, you'll end up losing the most skilled people first as it will be easiest for them to find new jobs. In the end, the best thing the team could hear right now to give them hope and motivation is something along the lines of "Look, I know your last project was brutal, but this one will be different. I want you guys fresh and effective, and that means not working yourselves stupid."