Stop frantically box clicking small groups of marines and use this simple trick to pull off splits you never thought were possible!
As as a group of banelings rolls towards your marines, patrol click them away at a slight angle. From there you can shear off smaller groups with patrol click in a Y or T formation. The effect is that the marines nearest the banelings will turn and fire, while the rest continue to run. This gives you maximum spread since each banelings hits very few of your units, allowing you to increase your cost efficiency dramatically. As a bonus, moving in patrol formation maintains a magic box, whereas move command tends to clump your units.
Banelings are coming! Patrol away at a slight angle after stimming.
Box select some of the marines and patrol click them in the first branch of the Y.
Do the same with another set.
The result is an extremely cost efficient split and trade!