I am a step parent, and quite frankly dislike the term but I am what I am. I have had the pleasure and good forune of raising 2 step children who are now all grown up and am in the process of raising two more. Some lessons I've learned may help you.
1. Allow the child to come to you. Make yourself available but do not force yourself into their life. Remember, you have just become competition. That's right, competition for mom or dad depending on your sex. Make sure you don't play that game. Give the child and your partner special time to be together and let him or her come to like you and out of that like will grow love.
2. Don't try and change all the routine in the house, curfew is a big one. Remember, you are not the parent and if you don't, be advised, sooner or later you will get told.
3. You should not be the one handing out the discipline. Feel free to assist your spouse in deciding if discipline is needed (behind closed doors) but allow you spouse to deal it out.
4. Show the child you love and respect his or her parent. This will be a key in gaining the child's love and respect as well.
Step parenting isn't always the nightmare of fairy tales: it can be wonderful.