My favorite way is to treat them as if they were my best friends--be sincerely, not cynically, kind and attentive to them. Even when they are being grumpy, respond as if they were being as sweet as peaches. Every time I have set out to deliberately show warmth and pleasantness to someone, they have responded favorably.
And guess what! Some of them have become friends. One of the warmest good-bye hugs I ever got from a departing coworker came from someone who'd been a champion sourpuss. I made it my business to try to brighten this person's day just by treating him with extra care consistently, day after day, and he turned around completely.