Warm pot, boiling water.
If you make tea in a mug, you've already lost.
The key is keeping temperatures as high as possible; that means a teapot as hot as possible before the (furiously boiling) water goes in. And then slapping a good thick tea-cosy over the pot. Which should be made of good thick insulating china. Or clay, or whatever it is.
There is only one problem here - as you whip your cordless kettle out of its stand just before boiling point is reached, in order to pour very hot water into the pot, the contacts will arc and destroy themselves. You must turn the bugger off *before* removing it from its stand. I think I destroyed about three or four kettles before I finally figured this one out.
And a +1 to Yorkshire Tea bags.