Stick to the specific issue that makes you mad. 

e.g. "I really hate it when you look at other men/women," 

versus

"You are such a flirt. You crave attention with that big ego." 

The hardest part is to stick to the original complaint. 

Do not follow your partner/child/friend down the rabbit hole with digressions and further, piling on, character complaints. 

Repeat, like a mantra, "I don't like it when you look at other people."