All Ceramic Restorations: Preventing Contamination of the Resin Cement at the Gingival Margins?

Dr. U. asks:
I have taken many courses on all-ceramic restorations. Generally the preparations are done on nearly virgin teeth with minimal caries. All the finish lines are supragingival. Cementation of the all-ceramic restoration is accomplished easily because the course instructor does not have to deal with saliva or blood. But in the real world, my margins generally end up subgingivally and I have to contend with tissue retraction, saliva and blood. How do you achieve adequate isolation so you will not get contamination of the resin cement at the gingival margins? I have found this to be a very difficult problem. Any thoughts?