Building a relationship with your stepchild will be easier when you've established a good foundation.
Say you want to be friends—not replace her parent
Addressing this common fear head-on will help her trust you, says Edward Teyber, Ph.D., author of Helping Children Cope With Divorce.
Let your partner talk you up
If your stepdaughter wants to go to the zoo, for instance, her parent might say, "Mary knows a lot about animals. How about if you two go and I meet you for lunch?"
Connect through his interests
If a preschooler loves dinosaurs, ask him if he can draw you a picture. With an older kid, who may feel powerless because a stranger has moved in, say you'd love to have him teach you something, like how to play his favorite video game.
Give it some time
If your new stepchild doesn't seem to like you, she may be having trouble getting close because she feels like she's betraying her parent.
Even if it works at first, it'll establish a pattern that'll be hard to break. And where will it end—with the 100th Pokemon card?
Discipline too soon
Leave the time-outs and reminders to your partner until you've been part of your stepchild's life for quite awhile—at least a year.
Bad-mouth your spouse's ex in front of your stepchild
If the child's parent misses an appointment with him, for instance, simply say, "It made me sad to see that you were disappointed today."
Give up too easily
Even if things go wrong, keep it in perspective. If she feels you're in this relationship for the long haul, she'll ultimately trust you enough to reach out.