Like the others in this thread have said, you can grow your email list quickly by giving away something for free to people who subscribe to your list.
Other than that one thing that also usually works well is to have a drawing or giveaway of some kind for the people who subscribe to your email list. Maybe offer some kind of gift card from a place like Amazon, iTunes or something that in a drawing for people who subscribe. This would help motivate people to subscribe so they have a chance to win the prize.
