It’s critical that most of us save and invest for our retirement. Don’t just save what you think you can and hope that it will be enough, though, as you don’t want to run out of money before you run out of breath. Take some time to come up with an informed estimate of how much you will need in retirement, so that you’re saving with a specific goal in mind. Here are three things to keep in mind, as told by our Fool contributors, as you try to figure out just how much money you really need for retirement.