After many mistakes I finally learned how to tell if your avocado is ripe to eat after visiting an avocado farm in California!
Simply gently roll your thumb over the tiny nub-stem on top and look inside the hole that it leaves behind. If it's bright green inside it's perfect to cut and use. If it's very dark or very gray it's old. If it's hard to roll off, it is still unripe, so leave it a while longer before cutting it. No more wasting avocados!
This page contains the following solutions.
When selecting an avocado look for one that is large all around and larger on one end. The skin should be nearly black. Gently cup the whole avocado and gently squeeze. It should be firm with a slight softness.
For years I thought if the sides were soft, the avocado was ripe. My husband heard on TV to touch the tip and if the tip is soft, the avocado is ripe.