You don’t need fancy technology. I would always write them on my hand. Then when I got done with an interval, I could just rub it off. Sometimes, I would pick up little pieces of stones per intervals and then toss them as I completed them – ie 5×200 – i would have 5 little stones.
Second thought print them out, and put them on a little piece of paper.
Or, these workouts are typically not crazy to remember (unlike some I’ve had from previous coaching). You could just remember them.