Any good method for the power down sequence?
Get 5 V and 3.3 V regulators that have enable, sometimes called shutdown, pins, then sequence them accordingly.

Properly sequencing the shutdown lines is not as trivial as it might appear at first glance.  Probably the simplest way is to use a tiny microcontroller, like the PIC 10F200.  This would have a single shutdown input, which then causes the individual shutdown signals to be driven accordingly in the right sequence with the right wait times between changes.