Make sure you write it down so people know whether it has dots, hyphens or underscores. For example, here is a Technorati search for ted2009 (over 200 results). And here is one for ted09 (just 3 results).

Here’s how the New England Chapter of ASIST asks attendees to do it. And here’s the principle in action at Flickr for Bath Spa’s Institutional Web Management event.

Enforcement is another matter, but at least if you let people know what the right format is, they are more likely to use it in their writing and when tagging blog posts, slides, photos etc.

UKOLN have a great briefing paper Use Of Social Tagging Services At Events.