I started by reading the entire Hibernate 3 docs. Then doing the Hibernate in Action demo. Then just
testing things out and writing code/mappings.
Then reading the offical and these forums whenever possible. Someone elses problem could always be my own.
Most problems you get you can figure out from the docs or reading forums. The more in depth stuff you are going to have to go through source code and their Jira issues.
There are lots of best practices you can learn from the Hibernate Caveat Emptor example and the Hibernate Wiki pages are nice.