Assalamua'alaykum wa rahmatullahi wa barakatuh,
To gain khushoo' (concentration) in Solaat is one of the hardest thing. At some point, when we are too preoccupied with the worldly matters, it will actually leads us to not being able to present our mind and our heart FULLY during our submission to Allaah subhanahu wa Ta'ala (in Solaat).
Some of the sayings of the Salaf as-Soleeh regarding how they try to gain khushoo' in Solaat:
When Hatim al Asamm was asked about his prayer, he said, "When it is near the time of prayer, I perform a perfect Wudu and go to where I am going to pray and sit down there until I become fully attentive to what I am about to do. I then stand up and pray, imagining that the Ka`bah is in front of my eyes, Paradise to my right, Hellfire to my left and the Angel of Death behind me. I imagine that it is the last prayer I am about to perform, stand up in hope (in Allah, His Paradise and rewards) and fear (from Allah's torment in Hellfire) and recite the Takbir while having full attention. I recite the Qur'an calmly, make Ruku` humbly, go into Sujud with Khushu1 and then sit on my left leg, with the left food laid on the floor and the right food raised up, all the while praying with sincerity. Afterwards, I do not know (nor feel certain) if that prayer was accepted from me!" [Al Ihya 1/179]
Bakr al Muzani said, "If you want your prayer to be of benefit to you, say to yourself, 'I might not have a chance to perform another prayer.'"[Jami` al `Ulum wal Hikam, p 466.]
Mu`adh bin Jabal advised his son, "My son! Pray the prayer of he who is just about to leave and imagine that you might not be able to pray ever again. Know that the believer dies between two good deeds, one that he performed and one that he intended to perform later on." [Sifat as Safwah 1/496]
Hammad bin Salamah said, "I have never stood up for prayer without imagining that Jahannam is before my eyes." [Tadhkirat al Huffadh 1/219]
`Umar said, while standing on the podium, "A man might have white hair in Islam (i.e. reaches in old age while Muslim), yet has not completed even one prayer for Allah, the Exalted!" He was asked, "Why is that?" He said, "He does not perfect the prayer's required Khushu`, solemness and attending to Allah with his heart." [Al-Ihya 10/202]
Saidina Umaar al-Khaatab radiyallahu anhu used to tremble when he recite ayaat of punishment in his solaat until he collapse. He fell sick for almost a week just because of that! MashaAllaah, how tremendously khushoo' were the Sahaabah.
Saidina A'li karamullahu wajhah used to weep everytime he came to the ayaat "Ihdina siraathol mustaqeem".
To let ourselves gain khushoo', it is also advisable for us to be khushoo' since when we are performing the wudhoo'. One of the sahabaah's r.a face will turn pale even when he was just taking his wudhoo'. It was because he has let his heart to be presence with him that he is about to perform Solaat and submit himself to Allaah subhanahu wa Ta'ala. (I cant recall which Sahabaah it was)
To gain khushoo' in Solaat isnt easy at all, but we can always try, try and try. Alhamdulillaah we have them (Prophet Muhammad s.a.w, the sahaabah and the salaf al-soleeh) as our examples on how to let our mind and heart to be present fully during our solaat.
And yes, one of the method is also to UNDERSTAND and PONDER on what we are reciting during our solaat (specifically to non-native Arabic speakers).
InshaAllaah, the more often we try to be khushoo' in solaat, eventually we'll be able to feel the bliss sweetness of submitting and supplication through our solaat. MashaAllaah.
Wa'alaykummusalam wa rahmatullah.