I always just watch the second hand on my wristwatch. But mental counting works pretty well too, once you get the rhythm.
Remember, Time wise - to go from 2 seconds to 4 seconds is only 1 stop. So even if you are a third of a second off, you are less than 1/6th of a stop off exposure wise.
And that buffer gets bigger the longer the exposure. (Not that you shouldn't aim for precision and consistency, just pointing out that it's not the end of the world... with most Neg films, that kind of buffer is PLENTY).