#1 I usually cut negative people out of my life to the fullest degree that I can - it doesn't matter whether they're friends, family, coworkers, whatever. I know it's impossible sometimes to get rid of them entirely (I am in a situation like yours where it is not an option) but a little cold shoulder goes a long way.
Also, every time your negative friend says something negative, just roll your eyes dramatically and say things like, "Gah, you're such a Debbie Downer!" or "Oooooookay, Negative Nancy!" or "Geez, drama queen much?" or even, "If you keep bringing your soul-suck to me, I'm going to have to start charging you for intensive therapy."
It's lighthearted and a little goofy, but it also draws attention to that person's negative behavior and brings their
attention to it. They might not even realize they're being so obnoxious (or they might not care, in which case see #1).
As far as positivity goes, I enjoy yoga or other forms of a good stretch, losing myself in fiction, lots of exposure to the natural world, meditation, writing, and surrounding myself with people who are chipper and optimistic as an antidote.