
Hi maysea,
I am having the same problem with my little darling :P although at the moment she is only 10 weeks old, she will be very large when she's finished growing ;)
Dominance isn't an issue, they do it because they get satisfaction from it, ie, attention. I have seen subserviant dogs to this. Even by pushing them down and saying "no" they are still getting what they want, albeit, negative attention. Mine is very persistant, whereas my other dogs learnt the down command fairly quickly.
When mine does it, I don't make any eye, hand or voice contact at all, I simply turn my back on her and walk away. She is gradually learning that there is no reward for her and doing it less, she still does it when she is excited, but I put this down to "puppy exhuberance". I am being very consistent with it as she needs to learn it NOW. I don't fancy teaching her in another 3 months time :D
HTH