Video store: "That's a great movie" (say something positive if you can... best to not say "that movie sucked, even if it sucked balls; Positive = better)
Grocery Store: Ohhh... fun one! Use the "broken wing" syndrome. Ask a basic question about a relatively complex cooking project, and make it clear from the question that *you* are doing the cooking. Why? 1) It gets the conversation started, 2) It shows you don't have a chick cooking for you, 3) It shows that you aren't living like a bear with a rotten apple and 3 six-packs of beer in your fridge and 4) Some chicks are *really* into helping/mothering guys a little bit -- it's the "broken wing" syndrome.
Know what's even better? If the conversation really gets going, and she's giving you signals, you finish with "Now I just need to find someone brave enough to take the first bite. I hope I don't poison anyone." Then it can go two ways... either she comes and helps you prepare it for the first time or she comes over to 'test' your work.
Funny story, but I tested that approach last weekend (I'm not on the market... it was more of an intellectual pursuit). And this was in front of my mother! Cute girl at the grocery, so I ask my mom what order I should put the ingredients in for my pot roast. Then I brought the girl into the discussion (she was right next to us). Did the whole "incompetent cook" routine... asked her if I could accidentally burn it... asked how long / what setting. Finished with "Now I just need to find someone brave enough to try it." And it was great... I got a smile, eye contact, and even a touch of the hair! Course I didn't go the next step, cause i'm not shopping for a female atm.