Read your salah (prayer) slowly. The khushu (concentration) will follow automatically. The Quran tells us that the essence of salah is remembrance of...
'...for without doubt in the remembrance of God do hearts truly find rest.' (Quran 13:28)
Frequent remembrance and glorification of God, seeking His forgiveness
All [religious] practice prescribes the remembrance of God, and to achieve this remembrance is its goal.