The trick with walmart, and well pretty much every other big store, is to absolutely avoid weekends. Thankfully I have a shift work job where I get lot of time off in the week, so I always do my shopping in the week. Preferably during normal working hours because most people are at work. If you wait till after 5 then it's going to be almost as bad as the weekend.
I miss when we had a 24hr grocery store, as I used to do my groceries late at night when there was not a single soul in there. I guess that's why they arn't open that late anymore. Our walmart used to be open till midnight, which was nice too for if I needed something there I'd just go later at night.
At least Shopper's Drug Mart is open 24/7. They have basic groceries and other commodity items there so if I'm stuck they're usually a good bet depending what I need. I needed distilled water for my UPS batteries a while back and I was able to get some there.