I keep having to take potassium because I keep losing it. What can cause this?

Diagnosis first. I would first have to ask why you know you are always losing potassium. The kidneys are very good at being able to hold onto potassium, especially when you aren't eating very much. Most people who are losing potassium are on blood pressure medications called diuretics (lasix, chlorthalidone, hydrochlorothiazide). A switch to a different medicine may be the solution.