I have a 4 and a half month old, so this won't be applicable for a while, but I would like to raise my son with the idea that he won't be an only child forever. I want my children to have a good relationship from the start. How do you prepare a toddler to be an older sibling?
One that worked well for us was for the new baby to 'give' a present to her siblings to say hi, and to let the elder ones hold her and give her cuddles from day 1 (actually from about 15 minutes old:-)
Before the birth, let them help to choose things for the baby so they feel part of the discussion.
After the birth, let them know how important their help is when Mummy and Daddy are tired - things like tidying up, helping cook etc are Big Brother/Sister jobs that the new baby can't do. This also helps them to feel wanted.
Young children (<5 years) need very little conditioning for the idea of a brother or sister. If they're socialized around other children with siblings, either in school, playgroups, or extended family, they'll learn this is a natural state. They'll just need preparation for when the little one is coming so they know what will change. Very young and it may mean they have to give up their crib or other personal effects. For older children it will mean just learning that they will have to share mom and dad's time (but they'll still have their full dose of love).
You won't plant that idea too firmly or early, though. There is always a chance that despite your intentions and efforts, he may be an only. It can be very hard to explain to a six year old why that baby brother never appeared.
Start early and talk often.
Talk about other people who have siblings. Talk about your siblings (if you have them).
For us, the thing that help the most was talking about the whole family relationship tree.
What is a brother, a sister, an aunt, an uncle. Explain what a cousin is, and grand-parents. Understanding that there are all these different types of family relationships helped us to set the expectation of what a brother or sister was much easier and then when they actualy came along it was simpler to transition.