Do you send a welcome PM or e-mail to your members (either manually or automatically)? If so, what do you say in it? Do members often respond to these, thanking you for your welcome?
I almost always send both a (personalized) private message and an e-mail message. By doing so, it increases the chances of them receiving and reading at least one of them, feeling a sense of warmth and community - hence, decreasing the chances of them registering for the hell of it and not posting at all.
I send them email/private message; on some forums I have this text customized from the vB default, others I haven't gotten around to it yet.
I just turned off the Welcome Email and customized the Welcome PM. Plus I use the Notices system within vBulletin.
I send the default welcome e-mail as well as a customized PM. Seems a lot of new members think I send them manually since they respond a lot of the time. That's alright. At least I know they're reading them.