Yup, exactly what Jeph said. Think of art the same way you think of say body building. There is no way you'll develop any sort of muscle tone by working out once a month.. at best you'll be able to maintain the strength you already had.
Drawing, or painting or any sort of art is about 90% practice. The other 10% is a natural ability of sorts, an ability to see things in a certain way. Even that will go away w/o nurturing though, and if you work really hard it's not even necessary.
Eitherway, study and practice.. a lot.. daily all day long. I rarely go an hour or two without drawing something (well except during sleep.. even then I'm drawing in my dreams
) it might be a hand, or an eye, or an arm in a certain position, a tree... just something, anything. It's constant and it has to be for anyone to improve.
The people you see with amazing art got there because of practice.. no other reason. Natural ability counts for diddly without constant practice.