First of all, as monsto pointed out, your child being eight means that they have a bit more emotional maturity, and so might be able to understand better.
Rainbowkids mentions that life for adopted children is often very different from ours here on the "outside."
- Life in institutions is often based on submissive/dominance models; therefore, your child at home may seem too aggressive or too passive.
- The extremely routinized life in institutions does not equip children with skills to handle transitions.
- Everything - smells, foods, sounds, textures, language, faces - is going to be radically different from what they are used to and recognize. Respect that by going slowly in introducing them to new things (people, places, toys, foods, etc.).
Explain these things to your child as best you can. Emphasize that their new sibling is joining your family after having lived with other people for a while, and that their old life was very different. Your eight-year-old should be able to appreciate that if they had to suddenly live full-time with, say, their friend's family would be difficult. Give them the opportunity to work it out and have them use their imaginations to put themselves in their new sibling's shoes.
About.com has a pretty thorough article. Their advice boils down to teaching your child:
- To give space.
- To ask (lots of) questions right away.
- The importance of confidentiality.
- The importance of role modeling good behavior.
- About good touch and bad touch.
- That you will listen to concerns.
- That you will keep things as fair as possible.
About.com also stresses you following through with your promises; it's important that you don't sacrifice your positive relationship with your biological child for one with the adopted child.
Sheknows.com has a list of tips for helping your current child bond with the new one. I'm presuming that your new child will be younger. The article stresses letting them get to know one another both in supervised and unsupervised environments. I really like the "building traditions" idea that the author suggests; it gives everyone a sense of future-motion.
Berkeley has a Parents' Network as well as advice about adopted children and introducing them into a home with a pre-established biological child.
FairFamilies.org has an article in their archives about this, as well.
ForeverParents has an article with tips, as well as other parents sharing their stories.
If you don't know how your child feels about having a sister or brother around, you do need to start talking about it. Because adoption has so much activity associated with it, it would be hard not to share the process with your child. After all, a social worker comes to your home, there is paperwork to complete, there may possibly be a trip to a foreign country in the planning stages or a visit with people called birthparents, and there is often a period of not knowing whether a new child will or won't be coming to the family. A child will sense that something is going on with this activity surrounding him, so you need to discuss it. The social worker doing your home study will want to know what you are doing to set the stage for welcoming the new child, and if you have thought about the sibling conflicts that could possibly result. The more your child is involved, the more likely he or she will be invested in the outcome. Some good hints specifically about adopting a second time but that apply to any adoption are presented in Sharon Kaplan Roszia's article, "Adopting Again: Talking to the Other Children in the Home."
This page features a number of stories and anecdotes from people who adopted after having had a biological child, the adopted children themselves, and others with professional or personal opinions. Obviously, it being a forum, it's less organized than an SE site, but the information is fairly clutter-free.
Finally, all of the advice I see is consistent about one thing: be fair to both your adopted child and your established one. As for preparing your biological child, be honest with them and try to give them the information in "bite-size" chunks. Don't condescend; instead, be direct and tell them that they should ask you and your partner/wife/husband/significant other any questions they may have about what's going to happen, what will change, what won't change, and what you need from them. Give children the ability to take some ownership, but also help them be understanding of the new face in the family.
As a last note: my roommate freshman year of college, Desi, had a younger sibling who was adopted. They were obviously very close, and he clearly adores his big sister. While I'm sure there was some initial troubles, their relationship was very strong when I saw them, and she didn't resent her sibling in the least.
Best of luck with the adoption process; my neighbors adopted and though it was a long, grueling process, they said it was worth it. Their son is a very beautiful, healthy, happy boy.