SmB6 is a strongly correlated mixed-valence Kondo insulator(1,2) with a newly discovered surface state(3,4), proposed to be of non-trivial topological origin(5,6). However, the surface state dominates electrical conduction only below T* approximate to 4 K (ref. 3), limiting its scientific investigation and device application. Here, we report the enhancement of T* in SmB6 under the application of tensile strain. With 0.7% tensile strain we report surface-dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be induced in epitaxial SmB6 films via substrate in potential device applications.