Teachers who give struggling pupils "lavish praise" could make them even less likely to succeed, research into classroom tactics has suggested.
The Sutton Trust education charity has warned that many strategies used by teachers have no evidence to show that they really work.
Too much praise for low achievers can "convey a message of low expectations".
Robert Coe of Durham University said teachers needed to know what was "most likely to be effective".
The study, What Makes Great Teaching, produced by Prof Coe for the Sutton Trust, drew on more than 200 pieces of research into what works in the classroom.
It highlights what it says are commonly used ways of teaching which are not supported by the research evidence.
Although teachers want to encourage underachieving pupils by giving them praise, this can have a negative impact on them, the study concluded.
It warned that research shows that if children's failure brings them sympathy they are more likely to associate that approval with underachievement.
In contrast, it said that pupils who are "presented with anger" will not have such a positive association with performing badly.
The study also challenged the idea that pupils should be put into different sets according to their ability. Separating pupils in this way "makes very little difference to learning outcomes," it said.
The researchers identified two main factors which are linked to whether pupils' results can be improved - the quality of teaching and teachers' subject knowledge.
The "quality of instruction" included "effective questioning" of pupils and a good use of assessment.
And it said that teachers with a strong understanding of their specialist subject were particularly likely to have a positive impact on how pupils learn.
"Great teaching cannot be achieved by following a recipe, but there are some clear pointers in the research to approaches that are most likely to be effective, and to others, sometimes quite popular, that are not," said Prof Coe.
"Teachers need to understand why, when and how a particular approach is likely to enhance students' learning and be given time and support to embed it in their practice."
Head teachers welcomed the study's emphasis on research-based evidence for teaching.
"We have long promoted the use of evidence to inform teaching in practice and the information from this report is a useful step in the right direction," said Malcolm Trobe, deputy general secretary of the Association of School and College Leaders.
Christine Blower, leader of the National Union of Teachers, said: "Teachers are all too familiar with the fads and fashions regularly promoted as the latest "formula" to improve teaching and learning, only to see them debunked and replaced by some other magic solution shortly afterwards.
"The fact is that teachers themselves are the professionals who best know their children and their students."