When To Do It
Use Mental Health Days sparingly. If you text-message your boss with the words, "Call me back or I'll jump" and you mean it, then that's the time to take one.
Don't plan them in advance. The best Mental Health Days are those arranged at 4 pm the previous day, when you feel like you'll have a stroke if you have to come in the next day.
Don't worry about how busy it is at work.
How To Manage It
Keep a mental list of those people you work with who've got either an ARM they're struggling to pay off or a taste for Stickley furniture. Those are the work whores who are always happy for some extra hours.
Call them at 3:50 pm and leave a message on their voice mail: "If you hurry, you can pick up some extra hours tomorrow!" Make it sound like a great opportunity.
When they call back three minutes later, act like they've gotten really lucky. Then call your boss back and tell her you've got coverage for the next day.
How To Do It
First, don't set your alarm. You'll wake up at your usual time anyhow, at which point you can either go back to sleep or make a larger pot of coffee than usual.
Second, don't shower right away. Save your shower for the afternoon or early evening, when no normal people would ever shower. You may, however, brush your teeth on your usual schedule. When you do shower, make it sudsy, hot, and take a long, long time.
Third, have a stash of good MHD stuff laid up. My stash includes at least one fashion magazine, some Filipino potato chips, and British chocolate. Ramen is good, as are eggs or frozen pizza. The trick is to have fairly caloric food easily available that doesn't take a lot of effort to prepare. (Note: things which alter your level of consciousness are entirely optional, but might not be a bad idea. There's something nice about ending a big Scottish breakfast with a small nip of Scotch or a half-pint of brown ale.)
Fourth, make a space on the couch. If, for your own sanity, you have to sweep and dust the room first, do that as soon as you get up. Then you can park yourself on the couch the rest of the day with no worries.
Fifth, do nothing that is not fun. If the recycling bins are overflowing, ignore them. If the cat needs to be flossed, put it off. Bills that arrive in the mail on your MHD should be filed immediately and not paid until the next day. If your idea of fun is going for a four-mile hike with the dogs, do it. If it's lying on the couch eating V-Cuts and Flake bars and reading Elle, then by all means, feel free.
How To Recover From It
There's nothing better than following an MHD with a day in which you get a lot done through other people's labor. Get your car washed. Get its oil changed or the inspection done. Get your hair cut. Take things to the dry cleaner's. At the end of the day, you can look at your neatly-checked-off list with a feeling of accomplishment totally unsullied by exhaustion.
In case you couldn't guess, yesterday was my Mental Health Day. I filed my nails, modeled a new pair of shoes for myself, then went out and had a steak. Today I had my oil changed, inspection done, and hair cut.
Now I think it's time for lunch and a nap.
The most important thing about MHDs is to ease yourself back into life slowly, of course.