Early on, be just mildly nice. Make them earn your niceness. They'll value you more. Corollary: An occasional glare may have more influence than a smile.
Be in the sweet-spot of enthusiasm. Don't be too flat, of course, but don't be too enthusiastic or you'll sound desperate or out of control. Remember when Howard Dean was the leading candidate for the 2004 presidency until, in his speech after the Iowa primary, he yelled what seemed like a war whoop. That single moment took him out of the running.
Consciously decide how much to bolster your conversation partner's self-esteem. Usually, it's wise to say things that preserve or boost your conversation partner's self-esteem.
But occasionally, it's more effective to shake a person up, even deliberately showing controlled anger. For example, if a coworker is habitually lazy yet complacent, it may, in the right circumstance be wise to firmly, with a hint of anger say, "Would you mind if I give you some candid feedback?" They'll usually say yes. Then say,
"I'm feeling increasingly frustrated that you're not holding up your end. I'm having to cover for you and even when I'm, not, the quality and quantity of work getting done just isn't good. I don't want a response now. I just want you to think about it. Come talk with me tomorrow. "Caution: Showing unbridled anger will be less likely to get the person to consider your feedback and more likely to just write you off as a hothead.
Be low-maintenance. In today's ever more pressured workplace, it is critical that you're seen as low-maintenance. Here are a couple of non-obvious ways to be so:
Think twice before offering unasked-for suggestions. Why might that make you seem high-maintenance? Even if your suggestion is good, your overwhelmed boss doesn't want to take on any more and so may feel guilty or embarrassed if he doesn't do it. Or if he does do it, you've added to his overwhelm. The best question you can ask a boss: "Is there anything I can do for you?" Be the antidote to a boss being overwhelmed.
Tone down your "I'm offended" antenna. Be sure that your being slighted isn't a slight that's too slight to insist that you need to "process your feelings" about it.