The latest issue of Planet Hockey (issue 3, see link below to download) is well worth a look and has an interesting feature on defending a drag flick.
#1 Prepare early and decide on a defensive set up. Many teams have set running patterns for a penalty corner. For example first runner pressures drag flicker, second and third runners defend the possible lay offs and deflectors and post defenders cover post.
#2 Point at pressure target (ie drag flicker at top of D) with hips and shoulders. Only turn your head towards the injector.
#3 Adopt a relaxed stance.
#4 Offsetting. This technique that some teams use to defend flicks sees goal keeper covering one side and runner and post defender attempting to channel flicker in the direction GK is covering.