Don’t criticize, condemn or complain. This just makes people want to justify the things that they are doing wrong. Sometimes you permanently bar yourself from having any meaningful relationship with a person because of a criticism. If you want to discontinue their undesirable habits, you may want to show them how that action can actually hinder them from doing something that is really important or enjoyable for them and give them suggestions in a friendly manner.
Give honest and sincere appreciation. Studies have shown that one great need of people is to feel important or at least feel useful. People have literally gone insane just to be in an imaginary world where they matter. So make it a point to compliment people for the good things that they are doing so that good deed will be reinforced in them. Making people feel good about themselves and the good work that they are doing is so much better than driving people insane, right?
Arouse in the other person an eager want. Instead of telling people why you want them to do something, show them why doing something benefits them directly or indirectly. People find it easier to work when they anticipate some kind of reward from their endeavor. On the contrary, when people do something begrudgingly because they were told to so by a person who has authority over them, their output tend to be mediocre. So instead of bossing people around, show them the mutual benefits to be gained from working together.
(Some wisdom I’ve gleaned from reading the first few chapters of “How to Win Friends and Influence People” by Dale Carnegie)