Short-term memory is easily forgotten because our brain doesn't "code" it. But it codes long-term memory. Also, there is limited space in our brain for short-term memory like the article said, but we can never run out of space for long-term memory.
My doctor told me however, that it is possible to improve both short-term and long-term memory, especially the latter. If we try to make connections between information, it will be transferred to long-term memory and will stay there longer.