I find the statement "last-ditch approach" sad. Residential treatment shouldn't have to be a "last-ditch" approach. It is because there are generally no funds to send someone to one of these facilities unless or until they commit a crime and are sentenced to one, your insurance company agrees to pay for, maybe, three weeks, your child is Baker Acted by the police and is required to stay in one, but usually for only three days, or the really "last-ditch" approach, you decide to spend their college money on one.
I don't have the answer on how you get your child into one of these places before you need it as a "last-ditch." I do believe it should not be your last choice. We as parents are often faced with difficult circumstances regarding our children that are out of our scope of capability. When this happens we should be able to find help for them without spending four years of their college savings on four months of care.
The best method is to be proactive right at the beginning. Find your local mental health organizations such as: www.nami.org ; www.nmha.org ; http://www.nimh.nih.gov/outreach/index.shtml
All of these places have information on support groups, information, and training to help parents understand and help their children. Don't sit back and hope it will get better, because without help it will not.
No one expects their child to have a mental illness, but it happens and the best method for a cure is like any illness, catch it early and treat it early. Don't hid it away, reach out to people, it is surprising how supportive they are and you will be surprised who is going through a similar issue, but they never told you. I feel like I have "come out of the closet" in the past few weeks with my son's mental illness and I am surprised that it is freeing.