Battery Stapler Horse Fail
Some people seem to think that the multiple words option e.g. "Battery stapler horse" is the way to go. But I saw the XKCD cartoon about this some years ago and tried to memorise it. I've probably seen it two or three times a year since (More often than I visit some websites I have to log into) I've never remembered it yet! If on all the websites I used I had a different three or four word password only a memory champion would be able to remember them. n.b. I note both Alistair and I got it wrong anyway! Its correctbatteryhorsestaple.
Now did I capitalise any letters... what about websites that insist on: numbers, varied case, a low maximum length....
Part of the answer is for websites to not insist on complex passwords unless it is appropriate. I recommend people to use as complicated a system as they can do reliably. I wish I knew the full answer.