Jeanne Jennings is a strategy and product development consultant specializing in interactive marketing, and I recently came across an article she wrote last summer on ClickZ. It may be a year old, but it's as true now as it was then. Here's her list of best practices (and the full article, here):

• Make It Prominent: If visitors don't see they can e-mail your content, they won't.
• Make It Visual: Words are great, but a visual can often get the point across faster and is easier for visitors to skim.
• Include Address Book Functionality: How many of your friends' and colleagues' e-mail addresses do you know by heart? Letting visitors tap into their own address books to retrieve e-mail addresses overcomes the most common obstacle to sharing information.
• Address Privacy and Copyright Concerns: Letting people know how you will and won't use their e-mail addresses and the addresses they provide will increase their comfort.
• Offer Senders Their Own Copy: If it's interesting enough to share, there's a good chance senders would like their own copy.
• Support Personalization: By personalization I mean a personal message the sender can include in the e-mail. Many organizations shy away from this from concern about what might be said about them, their product, or the recipient; they worry about guilt by association. While there's always the potential for abuse, it needs to be balanced with the increased effectiveness a personal message can provide.
• Promote Your Business Goals: Often companies look at "e-mail this" as a public service. In reality, it's a great opportunity to further your Web site's purpose.
• Pay Attention to the E-mail's Look and Feel
• Encourage the Viral Continuum: Don't overlook a second viral opportunity. Give the recipient an easy way to pass this link on to other friends and colleagues.
• Don't Recreate the Wheel: There are a number of off-the-shelf solutions for "e-mail this article" functionality. While your IT team may relish the idea of building its own, do a cost/benefit analysis to determine which solution fits with your timeframe and budget. Boxed solutions are usually more expensive but can often be implemented more quickly than building the functionality in-house.