Are there any tricks you know to get people to register on your site?
Do you turn off post viewing (registration required) for example?

turning off post viewing is one the absolute worst things you could ever do.
if people cant see what you offer, they will not join (and spiders will also be unable to add your content)

here are a few links to help you out (be sure to thank ted s. & rex_b for these :D):
http://www.vBulletin-Fans.com/showthread.php?t=8134
http://www.theadminzone.com/forums/showthread.php?p=52001
http://www.theadminzone.com/forums/showthread.php?t=7247

I personally only make it so they just cant post unless registered.

A good way is to go to a forum where they do, in fact, allow advertising. Preferably in your signature, but don't overdo it.

I don't do anything , just look for a niche, i have a automotive website with all european model drivers who drive different brands but with some model. And by keeping this forum really european. This make this forum without advertising around 1400 members in less then 2 years

turning off post viewing is one the absolute worst things you could ever do.
if people cant see what you offer, they will not join (and spiders will also be unable to add your content)

here are a few links to help you out (be sure to thank ted s. & rex_b for these :D):
http://www.vBulletin-Fans.com/showthread.php?t=8134
http://www.theadminzone.com/forums/showthread.php?p=52001
http://www.theadminzone.com/forums/showthread.php?t=7247
Yeah, I hate it when people do that. It's true that people leave, that's what I do.

How about allowing visitors to view forums and thread titles, but making them register and log in to view the content of the threads. There is a hack to do this at vbulletin.org. I use it on my site, but more for privacy issues. Good luck!