Be careful where you put it. The meaning of just depends on its placement in a sentence, especially when it is accompanied by not, or by negative verbs such as don't or wouldn't.
Many people say just not when they mean not just, and this could lead to misunderstanding, embarrassment, even hurt feelings.
Not just means "not only," whereas just not means "simply not" or "definitely not."
He's a trusted adviser, not just a friend means "He's more than an adviser; he's a friend, too." But He's a trusted adviser, just not a friend means something quite different: "I trust his advice, but he's no friend of mine."