Quite simply, you can't stop her! Her bladder is still quite immature and I am told from reliable sources (vet) that bitches bladders are not fully reliable until they reach about 1 year old. However, from years of experience, I would say that most bitches and some dogs continue to spontaneously wee when they get excited. Things will improve though.
All I would say is try to lessen her excitement on your homecomings. Keep very quietly spoken, make slow movements and don't make a big thing of her welcome to you. Difficult, I know because it's so natural to want to give them lots of hugs and attention.
Be prepared as well - if you know it might happen make sure she is left somewhere where you can easily mop the floor when you get home!