Hi Mahesh Bhandari,
This is indeed the million dollar question. In very general terms the essence of the issue is how to marry teaching methods to research findings related to the question which conditions contribute most (efficiently and effectively) to language acquisition (SLA). And the challenges here include among others the dynamics in the research results (the SLA knowledge domain is constantly under development) and the degree to which insights reach practitioners and get adopted. And then there is also, as you also indicate, the context in which the teaching and learning takes place.
Find some references below illustrating some aspects of this process.
And find in these 2 papers an attempt to translate a number of research insights on which there is reasonable concensus to design principles for teaching and learning activities.
Koenraad, A. L.M. & Westhoff, G. J. (2003) Can you tell a LanguageQuest when you see one? Design criteria for TalenQuests. Paper presented at the EuroCALL 2003 Conference, University of Limerick, Ireland. https://www.dropbox.com/home/Public?preview=LQuestDesign_Canyou.pdf
De Graaff, Rick, G.J. Koopman, G.J., Westhoff, G.J. (2007) Identifying effective L2 pedagogy in content and language integrated learning (CLIL)