If you want to grow your email list fast, have a contest. There's nothing like a contest and giving away lots of valuable prizes to inspire action. This goes for blog subscribers as well as ezine subscribers.
Darren Rowse at Problogger.net has done this numerous times very successfully. A quick glance at Darren's blog and you'll see frequent posts about how to win stuff. Generally you must take some sort of action: comment on the blog post, subscribe to email list, etc. Rowse has over 50,000 blog subscribers; I don't know the numbers on his email subscribers, but I bet it's not too shabby.
Currently there are two email subscribe contests going on that I'm personally aware of.
1. Big Jason at BigMarketingOnline blog is giving away over $40,000 in prizes if you subscribe to his list. And they are not crappy prizes. Some are worth thousands of dollars. Hey, I subscribed when my good friend and colleague Mari Smith let me know about the contest.
I'd be happy to win Ray Edwards' $4,000 website copy critique, or a $500 AMEX gift card, or a one hour private consult with Jack Humphrey. Or a Facebook profile makeover with Mari Smith!
2. Chris Garrett is having a birthday give-away, but you're only eligible if you subscribe to his email list. Chris is a fantastic writer
and he's not telling what his subscribers may win, (my mistake, prizes are listed here, and they are GOOD!) and when you do subscribe to his list you automatically get his ebook about writing killer flagship content. That's worth it right there. Read his post to learn WHY he recommends having an email list in addition to RSS subscribers.
As with all ethical people doing business on the web, they provide a way for you to unsubscribe from their lists if you feel the information is not serving you. But wait until the contest is over... :-)