In the beginning, here’s how I thought I could best help my husband parent his daughters:

1. Provide him with lots of parenting material. (Sounds reasonable enough — depending on whether or not your husband wants parenting material. Mine didn’t.)

2. Advise him what to say and how to say it during his phone conversations with his ex (number one on my husband’s top ten list of things I did to drive him insane).

3. When he blew a parenting call, let the contempt on my face show everyone my disapproval, then behind closed doors convince him to do it my way.  (Suffice it to say, those behind-closed-door discussions should not be used to ‘divide and conquer’.)
After fourteen years of step-mothering my stepdaughters, I think I’m finally learning my step-ABC’s. Here are three strategies I’ve found to be more successful in helping my husband parent his kids.
A: Be an Anchor during Visitations
In stepfamilies, a routine can be an anomaly. More often, we find ourselves being tossed about like a rowboat caught in a hurricane, especially as the kids become involved in more and more activities. Thus flexibility is an important rule of thumb. Yet too much flexibility can wreck some basic structural elements necessary to establish the stepfamily as a family. 

Think about the anchor. An anchor dropped from a boat becomes firmly set on the bottom. Yet the boat has a certain amount of leeway to move about on the surface according to the current and the weather.   

In the same way, stepmoms can be an anchor in their stepfamilies. Decide with your husband on a few values vital to the wellbeing of your family as a whole. Hold firmly to fundamental values like respect, trust, and getting along with others; and be flexible in other areas, such as scheduling, tastes in food, and preferences in activities. A stepmom who can hold steady to the important things, but go willingly with the flow on the rest will add strength and stability to her family, and sanity to her husband!
B: Be a Buttress during Conflict
The dictionary definition for ‘buttress’ that applies here is: “something that supports or strengthens” and the verb form is “shore up.” In times of family conflict, your husband likely faces pressure to satisfy at least three direct competing sources: you, his ex-wife, and his kids — a daunting task. You, the stepmom, will perhaps have fairly precise judgment as to the motives of the parties involved in the conflict. Your natural response will be to add to the pressure by arguing your case against him, the kids, or his ex-wife. But your best course of action will be to listen to your husband as he sorts through the parenting issues facing him. Then help him answer the right question. 
 Some wrong questions he might be tempted to answer are:

• “What will make everyone happy?” OR,
• “How can I get back at her (the ex)?” OR,
• “What is the quickest way to get _____ off my back?”

The right question is:

“What is the most responsible choice — first for my child, then for the others affected (you, himself, his ex, etc)?”

Once he has made his decision, support him fully, and hope for the best. Whatever happens, you will all likely survive. He will sense your respect of his decisions, and your trust that he is capable of making decisions. He will then likely grow in his respect for you, for your opinions, and for his own decision-making ability.