I strongly advise against using modafinil casually. I advise conditioning yourself to work hard when you are energetic. Don't get in the habit of being energetic and not getting things done.

If you're going to be experimenting with placebos, I would suggest pre-committing to work hard after taking a capsule regardless of the apparent contents. Then you can examine the differential impact of modafinil versus just pre-committing to work hard for X hours. (I also recommend deciding on a break scheme for your X hours in advance. For example, take a ~10 min. walk/internet browse every 50 min., or have a certain number of timed breaks you can spend at your discretion. This sort of stuff has worked really well for me in the past.)