In Derbyshire, young volunteers were sold alcohol 25 times in 280 attempted purchases since March 2009, just one-third of the number which failed a similar number of tests carried out a year earlier. In Merseyside, a 12-month test purchase starting in April 2009 saw 115 tests carried out, with just five sales a 96% pass rate.
"It is clear from our test purchase results that retailers are now taking more care when selling alcohol to ensure that they don't sell to under-18s," said Steve Massey of St Helens Council's environmental protection department.
In Leeds, in 315 attempts to buy alcohol over the course of the year to March, volunteers were served just 18 times.
However the responsible retailers' approach has yet to be recognised in Durham, where Trading Standards officers have demanded powers to make test purchasing more difficult because retailers are passing them too often.
Councillor Bob Young of Durham County Council told the Sunderland Echo: "It is clear that certain retailers are becoming very wise to our methods, which has made it necessary to review our methods in order to ensure our work to tackle the illegal sales is effective."
Trading Standards wants to raise the age of test volunteers from 16 to 17-and-a-half, and allow them to work in pairs and later in the evening.