Just ask them if they have any extra DVD cases. I lucked out and got the manager both times since some junior employees will be reluctant to give anything away without their bosses approval.
The first time I ever went, the manager said he had just thrown out about 500 cases in the nearby dumpster. Luckily, it was a 'clean' dumpster (no foodstuffs) and I cherry-picked some off the top. But at another store, the manager was very friendly and saved them in-store for me. Said he even did the same thing for himself and his buddies all the time. Now that he knows me by sight, he checks his computer for how large next week's shipment will be. For example, at that particular store, they are ordering 300 copies of 'The Grudge' next week (god knows why that many people would want to see it, but whatever), so it will be a particularly sweet haul of swag next Tuesday .