Deficiencies in potassium can be caused by excess vomiting, diarrhea, and potassium depleting drugs. One of the reasons that your body feels achy after vomiting is due to a decrease in potassium. Good sources of potassium include bananas (of course), potatoes, and orange juice. An interesting fact is that dandelion (the weed that runs rampant in summer) is actually the only natural diuretic that does not deplete potassium, making it a good alternative for people with heart problems or kidney stones.
While eating foods that are rich in potassium is ok for most, it is highly recommended that you do not start a regimen of potassium supplementation before consulting with your doctor.