At registration (and speaker submission of sessions) we ask them to provide their twitter handle. If its a speaker, we then announce the sessions on Twitter along with the speaker name. For registrants, we tweet they have registered and their registration number (count). We only post one message every five minutes, and each will include the event hashtag (#sqlsatXX). We also tweet when a sponsor signs up (though we don't capture a twitter account for them).
We also show a twitter feed on the event home page, so its good to try to have at least one message going out every few day. Events should monitor the hash tag and try to reply to any questions posted.
At the event it's a lot of fun to use a large monitor or projector to post the twitter feed on a common area wall.