Understand that truth isn’t absolute.
There is your point of view, which is your truth, and the other person’s point of view, which is their truth. Once you realize this, you will be able to share your views in a polite, non- threatening way.
Use the “I” form.
For example, say, “I feel frustrated that you have not responded to my e-mail,” rather than, “You never take my suggestions seriously.”
Use simple language.
Don’t beat around the bush. Be direct so that the other person knows exactly what you mean.
Joke around a little bit!