How to improve concentration?

Try playing games that are intellectually stimulating and which can improve your memory like memory games wherein you take a few objects for ex: 10 different objects and take a 30 seconds to see all and remember and put a cloth over them and give yourself a minute to recall all of them and write it down. Whoever writes the names of all 10 objects wins. This activity/game will help you concentrate on those objects by giving yourself a deadline for those many seconds or minutes, hence its challenging as well and improves memory. You can play similar kind of games with playing cards to.

Avoid triggers which you know that are distracting you. Keep your motivation in front of you all the time to help you focus on your goals and not let you deviate.
The story telling game works only when you are in a group. It effectively helps in increasing the concentration level. All that you have to do is to set out three items together and then start telling a story using all the items in the story. Tell the story to one another. This game will not only increase your concentration but will also strengthen your memory power.
Physical and mental exercise both are required to improve focus. Try solving puzzles, crosswords, sudoku, etc and try increase the level of difficulty if possible once you get a hang of it.