I've had a really bad experience, my dad cheated on my mom with his wife (NOT my stepmother) and she's been in our lives since I was 11 and my sister was 7. It's been really hard, but I try not to let that cloud the fact that many of my friends have had the opposite experience, including my DH who has a wonderful stepmom.
I'd say realize that in many situations, the kids are JUST beginning to deal with their parents divorce and do NOT need to immediately be introduced to the new person in their parent's life. That's not to say the parent can't spend time with them- but it feels like a slap in the face when the first time you see your father's new condo is the first time you meet your soon-to-be stepmother.
Include the kids in the wedding to the extent THEY are comfortable with. Personally, I know my sister and I had ZERO desire to attend or participate in the nuptuals, but it would have been nice to tell us they were getting married, instead of my Dad coming back from a trip to the Caribbean announcing that he eloped. Not good.
Don't make every single holiday about the step-parent's family. We NEVER go with our Dad to see his side of the family on holidays now. Every holiday is taken up with her side. So we choose to go elsewhere, and that's sad, because then we don't see our Dad much over the holidays.
Let the kids have time with just their parent, sometimes. And don't manipulate phone calls, either (ie, sitting in the room commenting in the background as if you're part of the conversation). If I leave a message for my father, let HIM return it.
Money. This has been the single most divisive issue. The parent(s) should be allowed to decide, within reason, where, when, and what to spend on their children. Within reason means not going into debt beyond your means, not having to take on extra jobs, and not spoiling the heck out of the kid. It means that if a parent has the money, and wants to buy their child a car, pay for college, help them out with bills, or WHATEVER- that' s their right as a parent. The spouse can certainly have input, but not control. This is different in each family because of finances but in my situation, where there is more than enough money to go around, my stepmother wants it all for herself. Not exaggerating!
Overall, if you don't like someone's kids, or at least, aren't able to be nice to them and not interfere with their relationship with their parent, you don't belong in that family in the first place! Being a great step-parent isn't always possible, but as the saying goes, if you can't be helpful, at least don't be harmful, and that's what I wish my stepmother could have understood.