I know this is an old question, but I wanted to throw this out there in case it helps someone.
Background of answer: I suffer from periodic clinical depression and clinical anxiety. I have good brain chemistry days and bad brain chemistry days -- and there's an entire spectrum of possibilities between "full capacity" and "don't even". So I have to deal with this question more often than I would like.
When other tips like those presented in answers here so far don't work, I do the following: 1) acknowledge to myself that I will be working in "slow-motion" and just accept it 2) Don't focus on "getting stuff done" but rather on "setting things up to make it as easy as possible to get stuff done tomorrow"; this can involve things that seem like busy-work. Clearing out e-mail, cleaning my desktop, making lists organizing tasks and breaking them down, planning what specific small steps to begin with the next day, tidying up my physical space.
Often, following this process I get a lot of stuff done in the form of tying up loose ends. Best case in that setting things up to make it easier to begin allows me to actually begin it today rather than tomorrow. But even if I don't accomplish much concrete work that same day, it makes it much more likely that I'll have a higher-energy day tomorrow, and be able to get off to a solid start.
And I want to reiterate some things that others have already said: exercise, mindfulness meditation, healthy food, enough sleep -- all of these things can make a huge difference.