How to develop concentration in salah?

Shaykh Mawlana Shah Wasiullah Allahabadi (Allah have mercy on him) said,

‘A lot of people inquire regarding concentration in salah. I tell them all try to develop concentration towards Allah outside salah, when this will increase then it will persist during salah also.

The pious elders improved their salah similarly. They recited wird for the development of concentration towards Allah in their hearts.
Therefore, all of them recited the blessed kalima لا اله الاالله محمد رسول الله excessively as a wird. This renews the faith and strengthens it.

In my opinion you should also recite it abundantly. InshaAllah, concentration towards Allah will develop in your heart and eventually persist in salah also.’

Majmua talifat Muslih al- Ummat, volume 3, page 41
Medinah e munawwarah