This is one advice that we unintentionally forget to live by. Because it is so much easier to notice people's shortcomings, right?
Luckily, I discovered a technique to overcome this tendency to be a nitpicker.
Every time you're tempted to judge someone, think about your most annoying traits and the instances where people tolerated them. Perhaps a time when you said something insulting and your friend let it pass, or when your things were messy and your roommate organized them for you, or the way you always talk about yourself while your sister patiently listens...(I'm sure you'll think of something.)
Once you're reminded of your own imperfections, it would be easier to accept other people's. It would make you realize that you're in no position to judge them for you have your own share of flaws.