In order to increase concentration in salāh one should remember only one thing, that nothing should be recited without any intention, rather every word that you recite should have a consideration and intent behind it.
For example after saying “Allahu Akbar” when you stand, then think in your mind that I am now saying “Subhānak Allahumma”, then think and pay attention that I am now saying “Wa bihamdika”. Keep concentrating that now you are uttering “Wa tabārak asmuka”. Just like this keep focusing independently on every word that you recite and then say “Alhamdu lillah” and pay attention to the wordings.
Do the same when you recite your sūrah as well. When you go into Rukū῾ focus that you are saying “Subhāna Rabīy al-῾Adhīm”. Hence, whatever you utter from your mouth, do it with attention. Do this method throughout the salāh.
Inshā’Allah through this your thoughts will not waver, and then within days it will become easy to concentrate and you will feel inclined towards salāh and will also start to taste the sweetness of salāh.
Hayāt al Abrār Pg. 237