It seems clear that 4E really wants the party to have at least four PCs, so that all four roles can be covered. The official rules give tips on what to do when you can only have three PCs, but when it comes down to one or two PCs, the advice is pretty thin. For a single PC game, the DMG seems to punt, saying something like "you should probably play a leader or defender" and nothing else.
I'd like to be able to run one-on-one stories with characters in a larger, existing game, so I don't want to hear "use a different system." The larger game already exists, and the characters already exist, too, as 4E characters.
So far, I've found only good online ses of tips (which I will provide below as an answer) but I am eager to hear others -- especially if they have been play tested. The ones I've found already appear to be theoretical only.