Onboarding is the act of instructing someone how to use a product and encouraging users to continue using it. It's essential in communicating the value of the product after user has decided to give it it a chance, thus it is a form of marketing.
Clearly offer a way to try the product out. Have basic demo assets ready so users can try the service without configuration.
Skype has echo user where you can test your calls and chat.
Simplify your registration. Only ask information you absolutely must know; frequently just email. Automatically generate sensible defaults and request additional details later.
LinkedIn: ranking of user profiles from novice to all-star, gamification.
Offer optional social login. Facebook and Google integrations should be enough. You may want to offer additional login styles depending on your target audience e.g. programmers always have GitHub accounts.
Mandatory tutorial. Enforce users to complete a tutorial that introduces the most fun or useful features.
Pinterest: makes you to search and create your first pin.
Show a clear path to completion. Uncertain and unexplained waits feel longer than known and finite waits.
LinkedIn, Facebook, Quora: shows all steps you should make when starting.
Show the maximal value in the minimal time.
Twitter: enforced to follow popular people.
Facebook: enforced to add a few friends.
Dropbox: suggests installing Dropbox on the computer, not using the web app.
Get user to spend money. Even a tiny bit, they will feel more attached to the service.