It's often said that men are better at compartmentalizing than women -- you're going to have to learn a lesson from the boys when you're a working mom.
When you're at work, be fully at work. Once that trial period you set for yourself is over, you can't spend your days with your mind half-worrying about the babysitter, or you're not going to be effective. Likewise, when you're home with your child, you don't want to be BlackBerrying the whole time.
It's tough, and it's not always possible -- if you're working on deadline or your child is sick, allowances can be made. But it's a good lesson to learn for anyone in our hectic lives -- when you come home, you're at home. If you have work you need to finish, wait until the kid's asleep and enjoy the hours you have with him or her. And when you're at work, no one expects you to ignore the fact that you have a new baby, but you will be expected to focus.