I Just treat monday - friday, 8;30 am-5;00pm as work hours, and do work related activity. It's not so much about motivation as it is a discipline. I am motivated to do well in my business, so, that's what drives me in the first place.
Being self employed has pros and cons, but, to me at least, it beats working for wages/salary hands down. I enjoy the freedom (though limited) more than a set 40hrs working for a boss.
Been there, tried that, doesn't satisfy me very much at all.