You will find below some excellent exercises to expand your memory Brain training, focus and concentration are cumulative and worth the effort.
Just like the longdistance runner, or the aerobic dancer, much as they build lung capacity by stamina, you can do the same by concentration and focus, build a capacity to stick with it longer.
1] PICK 13
In the game you must keep a number in mind as you progress through the diagram and perform mathematical operations on the square in the boxes you choose. [ For Example: if you choose to start with the 13 in the upper left corner you can go two ways, either by moving right, (multiply 13 x 3), or going down, (13 x 7)]. Your goal is to go from 13 to 181 in 10 moves. You can go in either direction, horizontally and vertically ( but not diagonally). You may start in either upper corner, however , and this is the trick, only one starting point is correct. 

2] Memory shapes
Below are 2 pictures that are similar, Diagram1 and Diagram2.Study the diagrams together for several minutes.
Then looking at only Diagram1 see if you can remember all the differences between the two diagrams.