The single most important thing I have learned, especially this year, is be prepared, don't panic. Maybe that's 2.
DH is a great guy, but he could barely get his fins on in a 3ft spring, holding on to a ladder. I then I noticed a leak coming from my 1st stage. He couldn't get it off because he didn't release the pressure and got frustrated. I had to take my rig off, fix it, and put it back on, while he was huffing and puffing about it. I realize he was out of practice, 18 months and a broken leg, but if I hadn't practiced the doffing and donning in a pool I would not have been able to fix the issue.
I will be getting him in the pool with a tank, and working on skills, etc. I also plan to take rescue class to further my "be prepared, don't panic" mantra.