The plus behind your own minus
The reciprocity principle says that we give back what we receive. If someone gives us something negative, we'll give them something negative back. If we, however, receive something positive, we'll be inclined to give back something positive. This implies that if we want someone else to give us something positive, we have to first make sure that we, as well as we can, avoid negativity and give the other person something positive.
That sounds simple but there is a complication. What do you do when you see someone doing something which your are actually very critical about? Do you have to keep silent about that and pretend everything is just fine? No, that is not what I recommend. It is good idea to talk about what you have observed. But before you start talking it is usually wise to transform your own negativity into positive questions and messages. When you see an employee doing something wrong (for example, he is repeatedly interrupting his customer), think about how you want this person to behave instead and how that would help. Then ask him how he might accomplish that ("Customers usually find it pleasant and important to have the time to express their thoughts freely. How can you help them to do that?").
It is not a problem when you think negatively about something you see someone do. As people we tend to be a bit more sensitive to negative information than to positive information. We do not have to feel guilty when we notice ourselves having negative thoughts. That is only human. It is also useful because it helps is discover opportunities for improvement.
But what we need to be effective in conversations is that we are able to transform this negativity into positive and constructive questions and messages before we start talking. The essence of this transformation is what I call finding the plus behind the minus. We take the negative thought seriously and explore what is behind it. Apparently the topic is important. How can we verbalize this concern or value positively? When we have found this out we can be more effective in our communication.
By the way, we do not have to be convulsive about banishing each and every negative word from our conversations. We do not need to go that far. It is usually helpful to be a bit more aware of the reciprocity principle and to take it into account at least to some extent. To the other person that can make a big difference.