Before going any further: I think that crunch mode is not sustainable and is best avoided. I won't analyze the phenomen or its causes, there has been much written about it. However, given it definitely exists, though usually for shorter periods of time. Having had this experience a number of times, I felt I should organize my thoughts for future reference (for myself, and maybe for others too).
Some of the points below might sound like good practice for software development in general - often we only start working on process problems after they've blown up into our face. Holding retrospectives after such periods can bring quite a change, so don't miss out on the opportunity! However, we haven't even started yet, so let's step back in time.
Knowing why the close deadline is important from the beginning is crucial to keep the team motivated during this period. Working overtime for no apparent reason makes the already bad situation worse.
Have well defined boundaries for both time and scope. This is generally a good practice, but this is a must - setting out on a death march without a clear purpose is unlikely to succeed. Break things down into small (4-8 hours) tasks, prioritize them (and I mean an ordered list, not making all of them into critical priority). If the added up estimates already run out of the possible available time, cut scope. Most likely the only way it can be cut is vertical - e.g.: we will have logging, but we will work against only a single library, and won't make it pluggable for this deadline. Or we won't make the logging destination configurable (though it's a contrived example, it helps making the point).
Taking on technical debt can help. Be sure you all understand that this does not equal to crap code, unconditional copy-paste, etc.. Only introduce debt knowingly (you might want to keep an actual list of the shortcuts you have taken so you know what to fix in the upcoming releases).
Stop experimenting and move back to the comfort zone. Almost all projects involve some new element that the team is not familiar with it. If the team is just learning unit testing, they likely write the tests after they got the code to work, and are taking a long time to write them. In that situation, not writing tests for the time being could be the right choice. However, be careful to get the right message across - you are not dropping the practice because it slowsdevelopment down, but only rescheduling the learning period to some other time. Also, just like with performance optimizations, make these decisions after measuring - don't do itbased on speculation.
On the non-technical front, you can cut back on meetings, both in numbers and in duration. You might also want to schedule them so they don't interrupt the workflow. Have a status in the morning and at the end of the day, with a team lunch if more discussion is needed.
Work from home. Of course, some infrastructure is needed, and it might not be an option for everyone, especially for the person where next door is a construction site. However, saving on the commute time might could give you some extra time during the day without cutting too much into your personal life. Significant Others are more understanding if you are a bit more tired when they get to see you at least.
[Scope section was updated based on @ljszalai's comment on twitter]