Give no more than 3 names
Anymore than a few names and the client perceives you're just passing them off on anyone, rather than hand-selecting people you think would work out for them. This is a reflection of the level of service that you provide to your clients - so even if it isn't "hand-selected" you can at least give them the appearance that it is, which will make them feel special warm fuzzies when they think about you even if they can't work with you for their wedding day.
Know the availability of the referral
There's nothing worse than finding out your favorite photographer is booked, only to find out that their recommendation for someone just as great is booked as well. Try to avoid sending your clients to dead ends, it simply attaches feelings of frustration to their interaction with you. I use google calendar to share my availability with other photographers and a syncing program that works with my preferred calendar offline so that my google calendar is always up to date.
Make it personal
Say something really nice or even a little boasting about the person or people you're referring. It's one thing to say, "This person is available" and it's a whole different level of referral when you can say, "Not only does this person happen to be available at the moment, but I would totally trust them to do my own wedding."
Know your referral's prices & style
While there are many photographers in any given area who may be available and wonderful, not all of them share your same price range or photographic style. Before you send on a referral you don't know very well, take a few minutes to figure out if they'd actually be a good referral for your client and if you'd want their clients being referred back to you. The ultimate goal is to work with people you love, so try to send your referrals on to people who can reciprocate the kind of clients you're looking for as well.
If you've never received a referral from another photographer, you need to give it before you can get it. If you don't have any referrals to give yet, you can start by contacting people with a similar style and price to see if they'd be interested in getting referrals from you in the future. 'Tis the season for giving!
Anne Ruthmann is a lifestyle & wedding photographer from Boston, MA. She spent 10 years practicing marketing & management in corporate and non-profit businesses before pursuing her passion for photography as an independent small business. She loves helping others find creative and low-cost solutions to business problems. Follow her on Twitter to see her daily adventures and thoughts.