My children have many overlapping diagnoses, and what works for one doesn't always work for the other, but here are some of the things we've found to help when adopting an older child with known PTSD:
Edited to add: Something we often forget is that we, the parents, can experience PTSD or Secondary PTSD. Working with children with trauma and/or attachment issues can be a very traumatizing experience. It can also trigger issues from our own past that we may have thought we had dealt with long ago. Please get help for yourself and take care of you! Parents get overwhelmed too. If we ignore ourselves and give until there is nothing left then we can no longer function as parents or even just as people.
Some resources: http://www.psychologytools.org/ptsd.html
Get a good therapist who understands and has experience working with adopted/foster kids with trauma. We love our EMDR therapist for our daughter, but still use a good attachment therapist too. Don't be afraid to "fire" the therapist if it's not a good personality match.
EMDR therapy is the most recommended therapy for people with PTSD. It is most often used by soldiers and victims of trauma (like rape or being in a natural disaster), and usually only requires 2-3 sessions. Obviously people who have suffered from long-term trauma (Complex PTSD), such as child abuse, would most likely require more sessions. There are no meds specifically for treating PTSD, but with good therapy and meds that help with the symptoms, the child can recognize the effects of the trauma, learn to cope, and move on to dealing with the cause of the trauma.
Don't be surprised if PTSD is not your child's only diagnosis, adopting older children usually means they have more going on then just one issue - attachment disorders, physical and sexual abuse, bipolar disorder, ADHD, FAS/FAE... caseworkers will not or cannot tell you all of what caused the PTSD, and often symptoms overlap and appear to be other things. I don't think I've ever heard of a child with RAD that didn't also have PTSD. Here's a good chart for Overlapping Behavior Characteristics.
Puberty will exacerbate the problem. Be sure you do everything you can to facilitate attachment before it hits and be ready for some regression.
Bedtime can be a huge PTSD trigger, often causing night terrors and nightmares. Our son frequently has trouble going to sleep, sleeping through the night, and during a particularly bad period, would wake up with his heart pounding and his body shaking all over (his central nervous system releasing stored memories). His "nightmares" of the devil were so real he thought they were visions. Hubby and I took turns sleeping near him for a week.
Be prepared for your child to blame you for their past. My daughter recently admitted to seeing one of her past abuser's face everywhere - on walls, and particularly - over my husband's face. She hates "him," rages at him, tells him he is mean and evil, accuses him of yelling at her (even though Hubby isn't even raising his voice) and dissociates.
When in a meltdown Kitty mentally shuts down - we call it fight, flight or freeze. She acts instinctually to protect herself. It's difficult not to hold her accountable when she rages during these times, but we've learned to wait until she's calm and then process what led up to the event so we can help her prevent re-occurrences. There are times when she doesn't remember the event at all.
Avoid stimulating events, situations, etc. We have our daughter and son's rooms stripped down to the bare essentials, not just because they've done lots of damage, but because anything more is overwhelming. We avoid big groups, noisy places, and in particular any situation that we know could trigger a flashback. I strongly recommend this, particularly for the first 6 months the child is in your home. Especially avoid the temptation to travel, have a party, introduce your child to everyone, shower him with gifts - he's already dealing with an overwhelming number of changes just by moving into a new home with a new family.
Some suggestions for dealing with schools. PTSD is rarely recognized in schools. My children have behavior issues as well as diagnoses that require an IEP (including OHIs -other health impairment- like ADHD). Once you get an IEP though, you can access behavior accommodations that can really help. Look for knowledgeable parent liaisons who will help you find the resources and accommodations your child needs. Be aware that no matter how wonderful and helpful the school seems, they will not volunteer information, and they may actually discourage you from getting help. Request an assessment of your child in writing - they have 30 days to respond. Make sure they know you are willing to get, or have gotten, in touch with a lawyer. ADVOCATE for your child. NO ONE else is as qualified as you are about your child's needs.
Read Beyond Consequences, Logic and Control. It really helped us understand why our kids act the way they do sometimes (they are SCARED!). I also liked Parenting with Love and Logic - it's for "neurotypical" kids so a lot of it doesn't apply to kids of trauma, but it helped me stay calm!
Get lots of support and take good care of yourself, your husband and your marriage.
Try to surround yourself with people who "get it." In person is best, but the internet has lots of great parent support groups, including this one http://forums.adoption.com/special-needs-adoption/ and https://www.facebook.com/groups/147916451954056/ . There are also great list serves (Yahoo groups that work via email - as well as online) and other supports such as http://www.radzebra.org/ and http://www.momsfindhealing.com/ .
Kitty - came into home at 11, now 13 - originally diagnosed with ADHD, ODD, learning disabilities, attachment "issues" and victim of abuse. Now diagnosed with bipolar, attachment disorder, ODD, Complex PTSD, learning disabilities, and victim of abuse.
Bear - Kitty's 1/2 sibling, came into home at 13, now 15 - originally diagnosed with PTSD, conduct disorder, victim of abuse and mood disorder. Now diagnosed with bipolar, RAD, ADD, PTSD, cerebral dysrhythmia, and victim of abuse.