More than once I read about writer's block. It seems like a problem that has no right answer, but once I read about Ray Bradbury (Fahrenheit 451). He says the best way to fight writer's block (and I do consider all creative enterprise much like it) is writing something completely unrelated. He had tons of stories he never finished, because when he ran out of ideas for a story he was writing, he just started a new one.
Now, slacking off is a relative term for me... Is studying something else or posting on a Forum (where you write, interact with other's ideas and read a lot of different stuff) really slacking off? I mean, there's a study about Creative Idleness (or "Ócio criativo", as I heard it in a brazilian university). It says that, sometimes, doing nothing productive can actually make you create better and faster.
I totally saw this in loco when I started working at a small tech company. My 4 hours shift was highly productive, I didn't make any stops, and I finished my tasks feeling really pumped to do a good job. When I changed to 6, and then 8 hour shifts, I couldn't focus as much, I had difficulty in delivering some of the work because I kept wandering off, specially in slower times.
So, yeah, try to be as productive as you can for short bursts, and not make a lot of stuff in a single day. It's a bit like exercising. Except there's no sweat involved, you don't need to pay to use equipments outside your house, and you don't get any health benefits from it.