Writing a statement of purpose?

I've started to look at applications for a second degree, and am struggling with how to approach a statement of purpose. One of the things that I'm not sure if I should address or not are my grades. When I initially started school, I honestly had no idea what I was doing there, and had terrible grades for the first several years (think Fs, Ds and Cs). Over the years, I became more self-motivated and for the past 2 years, I have been an A student, qualifying me for some of the programs I want to get into. I'm not sure if I should let the upward trend in my grades speak for itself, or if I should address it in my statement of purpose. Any other general tips on writing statements would be awesome!