In a rack of 8 ball, usually, your balls are all over the table. Luckily, in 8 ball you have the luxury of picking your routes and can, therefore, pot balls in clusters. Ideally, you want to make all the balls in one area of the table before moving on to the next to minimize cue ball movement. This drill helps train you to do just that. Apply the lessons learned here to your next game of 8 ball and you will run out more often.
Use less balls and work your way up to seven on each side. Start with the 3 closest to the end rails.
For each group plan your entire run in advance. Try to stick to that plan.
Run the balls in sections. Clear the first row (1,2,3) then the ball on the spot, then the next row.
Run the balls in order.
Only use corner pockets.
Get your problem balls that block other pockets out of the way first. This is also an important lesson for games. Sometimes you have to take your easy balls to open up pockets for the more difficult ones.
Leaving yourself only a small angle on each shot will make it easier to stay on one side of the table and not run into other balls.