We were born with 2 ears and 1 mouth - listen more than you speak... I found that this drives all other areas of interpersonal interaction: you need to understand where someone else is coming from / what their motivations are before trying to "fix" things or "pitch" them... sometimes people just want to be heard above all else

Honestly and simplicity are the best policy. Don't bs, don't overpromise, own up to your failure, and follow-through on the things you say you'll do (and it will be easy to do this if you are honest about what you can / can't do)

Favorite book: 7 Habits of Highly Successful People; the title sounds like a business book but it's really about how to be a better person / friend / parent / colleague / etc