I don't see myself as a vindictive person but just like my Momma said, "what goes around comes around." Maybe others call it Karma. I don't know but seriously, it's nothing to mess with.
Trying to stiff your vendor and not paying what they're owed, and then lo and behold, you want to hire them for more. Being the kind of person that says, "oh yeah, i have a great intro for you" and then crickets. Deploying passive-aggressive behavior all over the place and then expecting kindness and respect.
The list goes on. What goes around comes around. Pitching new clients, looking for a new position, growing your business, heck, it's all the same:
Play freakin' nice
Be honest and have integrity
Be grateful for all you have
Know your stuff
You don't have to "give it away" but generosity of spirit comes back to you when you least expect it.