Seconding the folks who've said the key to writing and drawing is just to do it even if you don't feel like it. I know this has basically been answered already, but I figured I'd step in and give my two cents anyway.
Right now, comics are my full time job. I typeset and color many of the Premium Tapas comics, on top of try to keep up with my own personal comic (which doesn't always happen because the stuff I'm paid to do takes priority). Am I always motivated to work? Heck no. Some days it's like pulling teeth to get myself to sit down to work. And it's almost never for any particular reason, it's just the nature of any type of work, no matter how much you love it. Most days it's super easy, and some days it just isn't. But I can't really afford to not work even when I don't feel like it, because deadlines would pile up.
So what I do instead of trying to find ways to pump myself up and get excited for work is I'll set smaller goals for myself. If it's being a hard day, I shouldn't try to work more than I realistically should, because then I'll just start fighting with myself. I used to do this even when comics weren't my job.
So lets say I want to get one page done that week, since that's what most folks who have personal webcomics do, but I'm really not feeling up to working. I'll take a look at what all needs done on said page, and break it up into pieces. If it's a five panel page, I might say, okay, I'll get at least two panels inked a day, and on the day I ink the fifth panel, I'll also start flatting. Usually what ends up happening is I'll start inking those two panels, but wind up inking three instead. But if I'm feeling especially bad that day, I'll stop at two. I still achieved my goal, so there's nothing to feel guilty over. I can go off and play video games or whatever else I do in my space time to unwind.
Anyway, I hope that helps. I've found the move I practiced this sort of thing, the easier it's gotten to just sit and work for longer periods of time and get more done, even when I'm having an off day.