Police should avoid restraining people in custody with suspected mental health problems, a report says.
It follows the death of a man in Yeovil in 2010 who had been detained under the Mental Health Act.
An Independent Police Complaints Commission (IPCC) report shows chances were missed which may have saved him, and makes a number of recommendations.
Avon and Somerset Police said it welcomed the findings and changes in procedures had already been made.
James Herbert, 25, was seen acting strangely after he had taken a "legal high" in his home town of Wells, Somerset.
He was restrained by officers, taken by van to a police station in Yeovil and left naked in a cell. He was later found in an unresponsive state and pronounced dead at Yeovil District Hospital.
An inquest in 2013 found he died from "cardio-respiratory arrest in a man intoxicated by synthetic cathinones, causing acute disturbance following restraint and struggle against restraint".
The IPCC report says police missed a number of chances which may have prevented his death.
These include opportunities to get immediate mental health support, to check on his wellbeing and to recognise a medical emergency.
The police watchdog has recommended officers nationally should use special techniques on people suspected of having mental health problems whenever possible.
They include prioritising the welfare of everyone involved including the patient, the use of containment rather than restraint and better sharing of information about people with mental health problems.
Earlier this month a misconduct hearing dismissed allegations that Temporary Inspector Justin French, who was on duty in Yeovil at the time, had lied at the inquest.
The Crown Prosecution Service ruled no criminal charges would be brought against Avon and Somerset Police or any officer in connection with the death or the evidence given at the inquest.
Assistant Chief Constable of Avon and Somerset Constabulary Nikki Watson said: "All too often the police service has been the service of last resort for people in mental health crisis when all else fails and the events leading up to James' tragic death is a clear example of this.
"This is why we have to work together with our partner agencies to improve the multi-agency response so that people in mental health crisis are given the support they need and deserve."
She added the force had made "wide-reaching and fundamental changes covering all the recommendations made".