I tend to plan out pretty far in advance unless I have a reason not to.
For consumables, random non-urgent wants, or things I know we'll "need soon" I'll research and then usually end up using something like 3Camels. This can apply to things like shelf-stable groceries, or business purchases like memory cards for my girlfriend's photography business (nabbed four Lexar 128GB 1066x cards for $60 a pop on a random price drop; normally $100 each). If the price drops, great, if not, I'll just buy when actually needed.
If I truly need/want something right away, there's little chance for that, and I become rather price-insensitive. I'll gladly overpay for lettuce on the way home from work if we've got a big salad planned, or pay a little too much for dog food if we're almost out, or pay way too much for another Wavebird Gamecube controller when Smash Bros. and Mario Kart versed people are gathering at my house. This sort of thing doesn't happen too often, and it's almost always due to insufficient planning or random chance.