Resolution #2: I'll tell my toddler to stay close at the store before we get there.
Why it's worth making: Children this age are seriously lacking in the self-control department, and developmentally speaking, that's totally normal. Reminding them of your rules only when they're on the verge of breaking them rarely works.
Your new approach: Do plenty of prep work. If you know your kid is going to challenge your limits in a particular situation, be clear about how she should behave ahead of time. (Remember, children this age like knowing what to expect.) For example, before you set foot in the supermarket, get down on your toddler's level and say, "You need to stay next to me while we're here. That way, we'll finish shopping faster and then we'll have more time to play later." Give her a couple of reminders as you shop, and be sure to thank her for being good if she doesn't wander away.