Almost as much as they need love, children need boundaries, and are adrift without rules. Learn to say (not scream, please) the following phrase: "In this house, we..." so that time together will not be bogged down with endless negotiations.
Corollary: "Let's get down!"
No matter how close in age you are to your stepchildren, you're still a parent figure; try to be an example of mature living and not 'one of the gang.' This is especially true if your stepkids belong to that group of psychotics euphemistically known as teenagers. Chances are they won't think you're cool for very long.