The symptoms of low potassium are weakness, cramping in the arms and legs, heart palpatations, and constipation. The best thing you can do to keep your potassium levels up, is eat foods that have good amounts of potassium in them like bananas, avocados, potatoes, spinach and oranges.
A glass of orange juice every morning is a great way to keep levels normal as well. The PP is right. I had dizziness, weakness in my legs and heart palpitations (doctor thought I was hyperthyroid) for quite some time before they figured out my potassium was low (it was on a routine blood test). It was just a little below normal, so it doesn't have to be real low to cause symptoms.