Best practices for SaaS user on-boarding?

What are some good practices and strategies for on-boarding users on a SaaS app?

Most of my users signup for the free version to try out the app. How can I tailer the on-boarding process to result in more conversions to paid plans down the line?

There are different techniques:

• Highlight some part of the screens and explain the features with a little textual description
• Force people to use features so they can learn about them (see Twitter onboarding process that force the new user to follow new people)
• Turn that to a game or step-by-step use of the app (see LinkedIn)

Lot of resources are available on the web on that topic, a few examples here:

• Quora: Onboarding: What websites have the best onboarding process, and why?
• Six Revisions: Onboarding Techniques and Examples for Your New Users
• UX.StackExchange: What are the good techniques to create a great onboarding process?