I think I've kinda figured it out for myself at least - and it starts the day before, even. I need to know that I'm going to work on X the next day, say "after school", then that'll be my mindset for the whole day. I get home, and I feel "yess thumbnailing time!"
The second thing, which is equally important, is to stay away from creativity killers. That's anything with instant gratification - my phone, twitter, news sites, netflix - because then my brain turns off completely and getting it back to work is really hard. That's why having the plan from the day before is so important, so I don't fall into that trap again.
A third thing that helps me is having a separate workspace for writing/comicking. In my case it's a little table, away from the computer where I do schoolwork and watch netflix. On the table is my drawing stuff, water, a little plant, nothing else. That way you have a visual and physical disconnection from your usual distractions and can get into creative mood easier! All it takes then is to just sit down there and my brain starts going!