Best tip I have is to buy an actual hourglass that measures 30 minutes. There is something about actually flipping a physical thing over that puts me in a work mode. Even if I get just a couple of sessions in during a day I find that overall I accomplish more.
An unexpected benefit is sometimes I don't need to start a session. I can catch a glimpse of the hourglass out of the corner of my eye and immediately enter a productive zone.