The best way to prepare skin for electrode application that I could find was to shave the hair the day before if necessary, and wash the skin with a damp towel or alcohol swab before drying the area off thoroughly. This will ensure that the electrodes don’t stick uncomfortably or pull at the hairs on the area the electrodes will be placed.
One other thing you want to do before placing electrodes on someone’s skin is to make sure there are no abrasions or cuts. Electricity takes the fastest way to the ground. If there’s an open sore or abrasion, it’s fewer layers of skin to go through. The electricity will prefer that and the patient may become uncomfortable before the data is taken.