The following article is by the Separated Parenting Access and Resource Center (SPARC)  and appears on DivorceNet.com:

When my stepson’s biological dad learned that I was to be the new stepfather, he told his son how lucky he was that he was getting two daddies when most people only get one! When the child came home from that visit and asked me if I was going to be his new daddy, right in front of his biological father, we started to correct him, not wanting to upset his dad. But the father told us what he had said to the boy about having two dads. That took us by surprise, but it did a lot to help us get off to a good start in a working relationship with the father. As a noncustodial parent, now, I greatly respect and appreciate the example set for me by this man. We both continue to have a positive relationship with the son we both love, even though I am no longer officially a relative. I strongly urge stepparents not to try to make themselves into "Dad" or "Mom" without the cooperation of the biological parent, it just gives the biological parent one more thing to get upset about (and rightfully so).

On the other hand, there is no better way for you as a parent to show the child that you love him and really have his best interest in mind than pointing out that he now has more family than most kids, not less, and how proud he must be to have so many people that love him. Divorce is hard enough on a kid without having parents that bicker over every little thing.

By doing this, you teach the child to look for the positive, to overlook minor annoyances, and to love without reservation or jealousy. You set an example that he or she can admire and grow from.

If you show your annoyance and jealousy when your child calls the stepparent "Mom" or "Dad", or when the other parent encourages the child to do so, then you show your child that you are insecure in your relationship with him or her, that you are afraid of being replaced. You also, in effect, force the child to chose between you and the stepparent, and convey the idea to the child that there is no way to be respectful and loving to one of you without the other getting upset.

This puts the child squarely in the middle of the bitter fight between you and the ex, and is not healthy for anyone involved. The pain of conflicting loyalties, indecision and of feeling ‘caught in the middle’ exacts a great emotional price for the child. It could eventually lead to the very thing that you are afraid of, when the child decides he or she is no longer going to pay that price and decides not to visit any more. It doesn’t matter whose fault it is, the child still pays the price.

Martial arts experts teach you to use the enemy’s weight and momentum against them. Instead of blocking their blows, you step aside and pull. It takes them off-balance and gives you the advantage.

It is essential that you work to avoid putting the child in the middle of your disputes. This can be extremely difficult when you are dealing with a manipulative opponent who doesn’t think twice about putting the child in the middle. There will definitely be times when you need to stand your ground firmly, but you want to keep those to a minimum for the sake of the child.

Learn when to yield, when to step aside, and when to be firm. Make it clear to the child that you love him or her, regardless of other circumstances. In most cases, a strong, loving relationship will do far more to prevent alienation by the other parent than anything you might accomplish by fighting with the ex. The ex wants to draw you into a bickering relationship. It drags you down to his/her level, and helps them to paint a picture of you as the bad guy.

You can’t control what the ex does, but you can control how you react to it. If you get upset at every little thing that he or she does, then you have given your enemy control over your life. If you relax and pick your battles carefully, you will be much more effective when you really need the leverage. You will also be much happier, as will your child.