"Does Salesforce.com actually work?" That is what some of your resistant users are asking themselves. They also ask questions like, "Why do I need to enter all of this information?" or "Things were working just fine - why change?"
As a Trainer and an Admin, you've been put in charge of answering these kinds of objections. The tricky part is that many of your users don't actually ask these questions out loud - most just don't sign in or update data.
So, how can we get everybody to "dance?"