Of course, the very best way to keep track is to measure everything out ahead of time before even going near the mixing bowl. This is called mise en place by the culinary pros, and yes, it's a very good way of making sure you don't accidentally add the salt twice or short-change the sugar.
We usually do this when making a new or very complex recipe, but with our everyday cooking, we tend to take shortcuts. We'll get out all the ingredients we need for a recipe and arrange them to one side of our work station. As we use what we need, we move the containers to the other side so we can keep track of what's been added and what is left.
If we get interrupted while counting out cups of flour or sugar, we usually make a few quick hash marks in the flour itself to indicate the number of cups we've already added. We'll also sometimes treat the mixing bowl like a clock face and position our measuring cup at the hour that corresponds to the number of cups (two o'clock for two cups, three o'clock for three cups, and so on).
What do you do?