Douglas White on Social Media Today published a great study on engagement on twitter and online forums.

What appears to be working extremely was:

• Threads were created as questions so as to elicit a response
• Forum entries were written in a conversational manner. Not all entries contained brand messages
• Replies to comments from readers were followed up and brand messages were included when appropriate
• The brand message was included at least once in all conversations

I strongly suggest you read the rest of the article, impressive numbers!