I totally agree with smzclark! I had a step-parent and I am a step-parent and it isn't an easy job by far. If you follow a few simple rules though, you may develop an amazing, unique relationship.
The two rules I have to keep in mind the most are...
1) The biological Parents make the rules, the step-parent simply enforces them if the bio parent isn't home at the time. Other than that, butt out. (easier said than done, I know)
2) When discussing the child, the step-parent should give their opinion (never call names or demean the child!!) regarding situations when asked, then back off. All you can do is support your spouse.
I love my step-daughter very much and I only want the best for her! However, I need to remind myself almost everyday that she isn't mine and I have boundaries she needs me to adhere to.