Inferring relationships between psychological variables in clinical samples and issues in generalisation
I don't know if you can completely remove the selection bias from such a sample. 

You seem to be referring (in some sense, at least) to regression towards the mean. This is one of the biggest (if not the biggest) problem with much research on clinical samples. 

What I would do (assuming resources) would be to take the same size (or larger) sample from the general population, and administer all of the same measures and tests to them. This would allow you to separate the predictors of the clinical outcome versus the variation due to other factors. I would probably employ matching from the clinical sample to the general sample (Gelman and Hill have a great chapter in their book about the matching, and that entire book is filled with helpful advice to analogous problems.

In my research, (on predictors of placebo response) I took large samples from the University where I was conducting my research on all of my self report measures, and compared my experimental population to them to determine if my sample were representative of the larger populatiom, which is a somewhat similiar approach to a common problem. 

Unfortunately, other than the Gelman and Hill book, I'm not aware of any literature on the topic (but that may be my own fault). 

I hope this helps.