4 finger rule works but I prefer my method.
Basically, write down (or visualise it in your head) ECTM.
Next, ask yourself are you climbing or descending? If you are climbing you are going from low altitude (-) to high altitude (+)
-ECTM+
So as you climb at constant TAS, for example, to the right of the T there is a + so that means mach increases. To the left is a - so that means EAS and CAS decrease.
Switch the - and + around for when you are descending.
Never let me down.