The NIST/NCJRS just released its manual for handlers of evidence. The handling of evidence is a material issue in many criminal and other trials.
Those who handle such forensic evidence must abide by strict practices or they jeopardize the validity and credibility of the evidence and the law enforcement agents and officers involved in the investigation.
The importance of such issues cannot be over-emphasized and are naturally the subject of a defense lawyer's cross-examination at trial. These procedures and best practices are fully described and detailed in the link below:
The Biological Evidence Preservation Handbook