I've written a couple times about asking for referrals.
One of the easiest ways to ask for a referral is to always have your business card handy, even with associates or business "friends" that you see somewhat regularly. Then, when you are saying goodbye, give them one of your card while specifically saying - "I know you have all my contact info, but could you do me a favor and pass my card along to someone you meet in the next week that you think I could be helpful to?"
This is an easy way to ask for help without making the other person uncomfortable about saying no or thinking of someone on the spot. If they really don't want to refer, all they need to do is to toss your card when you're no longer around. And if they do pass it on, odds are they'll tell who they gave it to.