Do remember that the process of becoming a family won't ever really be over, so don't beat yourself up about not being The Brady Bunch automatically. It's a process.
Don't expect yourself to immediately love your stepchildren. In fact, you may hate them for a bit.
Do let yourself feel selfish, jealous, or resentful, and don't feel guilty for having these feelings. Feel them, and perhaps discuss them with your spouse or a trusted confidant, but DO NOT act on them. You're in a very complex situation, and any negative feelings are both natural and expected.
Don't force bonding with your stepchildren by planning huge outings and showering them with attention and presents. Put yourself in the kids' shoes. With warmth and understanding, acknowledge that this new arrangement may be difficult for everyone, let the children know you want a relationship, and then back off. Just be around to offer your time and attention, and wait for them to come to you.
Do take the time to observe your spouse and stepchildren together to get a sense of their unspoken "family rules". How do they communicate, how do they eat meals together, how do they handle conflict? No two families are the same. Ask questions about things that aren't clear to you.
Don't take it personally if you are ignored/hated/considered a mild annoyance. Remember, these kids are in mourning and feeling very vulnerable. It's not about you- it's about what you represent.
Do propose small activities with your stepchildren. Things that keep your hands busy are good, because when children are distracted, they let their guard down. Making food, bathing a dog, putting things together, planting things, even being in a car together are some great conversation-starters.
Do keep a united front with the child's biological parents. Yes, BOTH parents. If possible, a disciplinary structure should be set up among all parent-figures involved, so the children have consistent boundaries in both homes. Everyone, including the children, should be aware of their expectations, and any parental-figures involved should stick to those expectations. You may disagree with a rule or punishment, but that's to be discussed away from the children. A lot of new stepparents fall into the trap of letting children disobey household expectations in order to gain favor with the them. If a kid sees a chance for some conflict between the two of you, he/she will exploit it. Kids are crafty.
Do take very careful, slow steps into the role of disciplinarian with your stepchildren. Help your spouse create a firm set of boundaries, because kids need to feel reassured that they will still be protected by their parents, but the bio parent should take the lead in explaining the boundaries and doling out punishments until your rapport with the children grows. This can sometimes take months.
Don't sacrifice alone time with your spouse just because the kids seem needy. A united front requires adult time alone, so put it in the calendar and make it a priority. A house cannot stand on a shaky foundation.
Don't EVER badmouth the other bio parent in front of your stepchild. If the child badmouths his own bio parent, your response is "Your mother/father loves you very much." If the bio parent skips a visit, doesn't call, or drops off the face of the earth and the child asks you about it, your response is "Your mother/father loves you very much". This is your party line.
Do remember that kids press buttons on purpose. They will lie to you about the other parent to you, and they will lie about you to the other parent.
Do remember to pick your battles when you start parenting your stepchildren. Having a clear set of expectations helps, but sometimes disciplining a child can be more about a general frustration than about whether or not Bobby's bed is made. Stop yourself, examine your own feelings and thoughts, and make sure that your reactions are appropriate.