Fill pockets with stuff. Suggestions include at least one high tech tool made by combining two previously unrelated bits of technobabble, one pocket-sized book with a charmingly eccentric title, some form of soothing candy, string, bits of metal, and anything plot-device-shaped.
Stand in front of a mirror and practice a mad glint in your eye.
Talk quickly and with authority. This will allow you to take charge when you have no actual right to do so, no logical reason for being there, and indeed no first or last name.
Nothing should surprise you — up to and including dinosaurs battling Daleks in downtown London — except human behavior.
Drop names, especially geniuses who have been dead for centuries.
If you're on any sort of mood-stabilizing medication, go off it.
When you make mistakes, make sure they're huge. Anything from "We're all going to die," to "I may have just begun the unravelling of space-time," will do.
Surround yourself with attractive girls and the occasional annoying boy. Their fields of competence should be highly focused, if not absent entirely.
Lastly, a long scarf will help, especially in America.
The actual post is here, and the comment threads are as good as the post, so I recommend checking them out as well.