Called  a "concentration chart",  this is an exercise you can practice away from the instrument that improves concentration and helps the brain "get into the groove" on those days that you just can't seem to focus. Don't laugh until you try it.  The point is to add facets to this chart to make it more challenging to perform. 
13 25 42 17 8 30
5 51 38 22 44 7
20 35 47 11 3 19
6 16 28 9 33 37
55 23 12 46 32 2
17 34 46 50 3 25
6 31 28 43 18 57
48 14 26 39 4 52
HOW TO USE A CONCENTRATION CHART:
Always use a metronome set at a comfortable, but challenging speed.  For the first run-through simply recite each number with a variable set for the entire chart. (i.e. subtract "2" from each number - the first line reading " 11  23  40  15
6  28 " and so on.)  Once that can be done smoothly at a respectable tempo, move the gradient of difficulty up a notch or two.  Read the chart in reverse (beginning at the bottom right) and/or make each cell alternate between "+2" and "-2", or any number you wish. (in this case, the first line reading " 15  23  44  15  10
28 ").  Then try reciting a color (instead of the number) on all squares bearing the number "5".  After this, difficulty can be raised yet again by overlapping a condition to, for instance, only the ODD numbers. (a condition being perhaps a finger snap  or a clap on the odd number only) Now that you have an understanding of how this works, you can tailor charts like this to your own needs.