In the documentary I watched two theories were proposed as to how Venus expels heat the first was that it has lots of vents or volcanoes that constantly burn it away, this seems to have been proven somewhat recently if Wiki is to be believed.
The second, regarded as spurious by some scientists, is that the crust sheds itself every so often when the pressure gets too high and the stuff underneath cools and makes more. I liked that one better just because the idea of a planet shedding it's crust was cool.