A nice PNF style stretch to do while you rot in the office.
Also, do it both ways.
- resisting the the knee, while pushing it up (as shown in the video), then press it down into the stretch
- contracting the glutes, pushing the knee down for a couple of seconds, then lean forward & pull your body towards the knee (maintain a flat lower back!)
Check out more drills on Mobility 101.
Update: Kelly Starrett also had a MWOD about that. Thanks Big Fudge.