Good suggestions. I find that if I'm really run down, it's best not to force it, but it helps to have a routine.
I write now whenever DD naps, which is not at the same time every day and usually not at my best writing time, either, but my brain is really and truly mush by the time she's asleep at night, so I only read then.
It helps to have a routine. My current one is to nurse DD to sleep (she's 10 months old) then make a fresh pot of tea and/or lie down on the sofa for 10 minutes. Then I get a bite to eat (ideally a small piece of really good chocolate) and get to work. My current goal is to spend at least an hour a day actually writing.
The other thing is to start from zero every day. Suppose your goal is a thousand words per day, and yesterday you had a cold and didn't even open the computer, or only wrote 50 words. Today's goal would still be 1000 words. Likewise if you had 10 shots of espresso and wrote 5000 words at a go, your goal is still 1000 words.
So, my answers to the original question are: catnaps, caffeine, and chocolate. A change of location can also help, like going to a cafe or the library.