My problem is that I'm not from London, or even the UK, so I can't really pick and choose my dates or buy much in advance (I assume the standing tickets are bought directly from the box office? will check their site to make sure). I'll try my luck with the Monday thing (I'm aiming at the first week of April), or will head to the BO when I arrive.. or I'll just go and see something else.
If you go to the Donmar website, you can sign up for Front Row info, they'll then email you on a weekly basis with a reminder for what week will be going on sale the following Monday as well. I think you can buy the standing tickets via the website.