In my opinion some of the best practices for SaaS user onboarding:

1. Remove anything in the way of your user reaching their "AH-HA" moment. Find a way to get users to see the value in your app in the shortest amount of time.
2. Encourage the referral. Find a way to integrate co-worker invites (because friends don't usually apply for SaaS apps) into the onboarding process. That way if they don't like your app someone else in the company is possibly still using it and may like it.
3. Follow-up. Since SaaS users take longer to convert, you need to do everything you can do shorten the cycle. You can do this by sending an automated but customized follow-up email. It can be as simple as asking if they need help or building your app to see where they left off and send a "helping hand" email to show them how to use a particular feature in your app. Here are some examples of follow-up onboarding emails: How to Onboard Users with Email, SMS, & Push
4. Ask very little upfront. Maybe just a name and email at first. Let them use the app and ask for more information as they use the app. Most people just want to look around and see if your app is a fit. A long form will deter many users from even trying. Remember, users can become paying customers without you ever know their company name, title, etc ;)
5. Tell users what to expect. If registering on your site is 3 steps and realistically takes 30 seconds, make sure you let your users know. People are more willing to stick with it when they know what to expect.
6. Give them another way to contact you. Don't lose a customer because users can't ask a question during onboarding. Try to always show 2-3 ways to contact you during the entire onboarding process. Ex: email, phone, text message, twitter, instant chat, etc.
7. Cut the cute stuff. Remove all the fluffy stuff. You'd be surprised at how many of your new users don't get your humor or just don't feel like dealing with it right now. I'm not saying remove every bit of personality. But if your tone, language, images, buttons, and anything else on your site is unclear on your site, remove it. Remember Clarity > Clever.
8. Don't ask for credit card first. If you were a door-to-door salesman and the first thing out of your mouth was "I have an amazing product you'd be interested in, but before I let you see it or try it out I need to hold your cash first" you'd have to quit. Asking for credit cards upfront has been proven not to convert well for many companies...so don't do it. Besides, do you really want a payment form as their first impression of your app?

There's more, but that's just the stuff off the top of my head.

Just remember to get eliminate everything that makes it hard for users to get right into your app and you'll be fine.