For us ( http://www.startupdaybook.com ) a few things have been very effective -- but it did take a lot of trial and error to find those few things.

BetaList and Reddit have been a big driver of signups as they have the exact audience we are looking to help (startups) and a wide audience.

Blogging has also been a big part of getting signups. Creating helpful posts on Medium with a small product mention at the bottom brings in a good amount of the right people.

Twitter is great for driving traffic to the blog posts which then in turn gets people interested in us and our product. Buffer is awesome, and Quuu is a great service that has a community retweeting your tweet for a nominal fee.

Overall our landing page conversion rate is 29.8% right now, so driving the right traffic really matters.