The motivation comes and goes. But I write whether I feel motivated or not... most of the time.
As far as keeping it up-- I have ideas while I go about my business (the more mindless the task, the more ideas I have), and when I do I text them to myself so I won't forget. Then I go to the computer and flesh them out when I get the chance.
Seriously, when I first started to blog I thought I would run out of anything to say in a week or two. I guess I was wrong.