Its like any other aspect of flying.....practice makes perfect (or close to it). I was a QHI in the Army and nearly every flight ended in an EOL. I practiced and taught them from every conceivable altitude, speed and configuration. It is extremely important to build a student or experienced pilots confidence by allowing them to practice the ultimate emergency.
The only EOL that I did not, under any circumstances, practice was the EOL at night. I did however teach the technique, but during day-light hours only.
I found the most difficulty with EOLs onto snow covered ground or static water.