I typically straddle the top tube and either grab the hooks or the tops for a full tuck. Alternatively, I can position myself nearly the same while sitting in the saddle, which gives me control on winding mountain roads.

For those of you who don't tuck: It's really cool when you get nice and aero. On a mountainous descent you can feel the difference in speed. it's like putting on afterburners; you could already be going very fast, then you go into a full tuck and you will accelerate noticeably.