Tuck the tail bone but in a relaxed way, forcing it is just as bad as not doing it.
It naturally tucks in if you relax your waist and lower yourself so that your center of gravity is near the navel area.
Also, the MOMENT you lift you chin at all, you lose your center.
Try this, have someone push down on your arm while you resist lightly with your chin down, unless they lift your arm, they cannot push you .
But soon as you lift your chin, bam! you can go flying, as you lose your root.
Raising the chin moves your head back enough to upset your balance.
For the very best place to read CLEAR and very workable ways to learn to achieve rootedness go here and read the articles posted:
http://www.iay.org.uk/internal-strength/peng-index.htm
You should print them out and read them til you memorize them!