People have a status quo bias, and don't like change. The problem with most efforts to deploy self-service, is that they're driven by IT, with the intention of reducing costs, rather than being driven (with the customer) to improve the service experience.
Some of the best deployments I've seen have involved feedback from customers/users from start to finish. They focus on what the customer/user wants to do and see via the portal (FAQ's, Knowledge, videos etc). Customers usually want a simple way to say "Something's broken", or "I need something" and be able to see what's happening with their requests.
The experience should be better and quicker than email. However, regardless of how good it is, you'll still need to promote the portal and an easy way to do this is with a soft launch. Get service desk staff to say "Did you know you could have logged this via self-service?" After a while, they can say..."Is there any reason you have not logged this via self-service?" The message will need to be consistently reinforced.
At the same time, the "What's in it for me?" messages need to be marketed to users, e.g. Self-Service is open when we're closed, You can access popular FAQ's 24/7, etc. Although I accept that each industry is different, people are generally the same, and will use any channel that's easy to use, and delivers the best experience. Changing their current habits isn't easy, but it's worth it for both the customer and the service desk.