Somewhere in year two or three, I accepted the fact that my stepkids wanted to spend some time with their father alone, and my presence was preventing them from doing so. Therefore, on some weekends, he did things with his children and I did activities with mine. This arrangement worked well for our children, but my husband and I began to feel like we were living separate lives on visitation weekends. We grabbed quick moments alone and reminded our ourselves that it was just for the weekend.
The other challenge I faced being a part time stepparent was the urge to fix his children when they were with us. Early in our marriage, I had a long list behaviors that I wanted them to change and considered it my job to assist them in doing so. After being unsuccessful, I finally realized I was being judgmental about his children and the way he and the biomom had raised them. Being a good stepmom and support to my husband meant not criticizing how their former household operated, but allowing him to parent his children the best way he knew how in those few days they were with us. I learned to let things go and look the other way so I would not be constantly trying to change who they were when they were with us. I can't say it was easy, but eventually I was much more relaxed (at least I think so) when I quit trying to fix whatever I thought they needed to do differently.
In speaking with other stepmoms who stepparent part time, they have agreed that sometimes you move three steps forward one weekend and four steps backwards two weeks later. The key to success is not to take anything personally and do what it takes to make the weekend work for not only the children, but you and your husband. Happy part-time stepparenting!