There is an interesting study, recently published, which explored what factors contribute to success in the stepfather/stepchild relationship. (For this particular study they only explored stepfathers, not stepmothers)
It seems the most important thing is making sure the child feels like they can express any concerns they have about the new relationship.
From the NYT Motherlode Blog
Professor Shafer described two mistakes couples commonly make in the transition: acting as though nothing has changed, or leaving the majority of the active parenting to the mother (although when the stepfather assumes too much authority too soon, children also report feeling frustrated). Children want to see their mother and stepfather get along, but that alone is not enough — they also need to feel that they can still count on their mother to hear and respond to their feelings about their stepfather and their new family. That, the researchers suggest, may make children less prone to hold resentful feelings toward their stepfathers, and more able to find a connection.
You can read the full article here.
Any parents of step-children want to weigh in?