Usually I prepare some work before I start (not do some work, I just prepare it) and keep the according windows open so they remind me of what I have to do after I stopped playing.
Of course, if it's something that's heavy on resource usage (as an engineering student, I work with CAD software a lot, and that stuff eats RAM for breakfast), I close the program and just leave the explorer window with the corresponding files, a list of specifications or some sketches open so I know what to do.
Then, I get a glass and a bottle of water, sit up straight (no need to ruin my back), take off my glasses (I'm nearsighted, and it's just enough to replace AA at normal screen distance)and find a comfortable position with all control devices in range.
Then I start the game.
For console ports that are comfortably playable with a gamepad (Overlord and Overlord 2, in my case), I sit as far away from the screen as the cable allows, and keep my glasses on.