My girlfriend tells me I have the magic touch because I usually pick out good ones. The issue I run into more than anything is how volatile any given produce department can be. Some places will only keep rock hard ones stocked, or maybe they have some regular early morning shopper who religiously buys all the ripe ones before you even get there.
If you’re the sociable type, schmooze with the produce folks at Kroger/trader joe’s and they’ll probably be able to tell you when the best time is to come, or maybe they can be assed to leave more not-rock avocados out.