Get into a habit that you only do while doing "serious work". For me, it was listening to music, on headphones. When I start doing "serious work" I put the head

Get into a habit that you only do while doing "serious work". For me, it was listening to music, on headphones. When I start doing "serious work" I put the headphones on. When I'm distracted, I take the headphones off. Repeat. Now, a few years later, I can get into Serious Work Mode, just by putting on the headphones, without listening to music. I've accidentally Pavlov Conditioned myself. It's great.