Learn about Design Patterns, they will force you to really use Java's features such as Interfaces and Abstract Classes, accessibility modifiers (public, private, protected) and others. That is the method I use when tutoring individuals in similar situations as yours that want to get back into Java specifically.

I recommend the Head First Design Patterns book if you're not turned off by the less-than serious nature of it. I really did not like the book at first glance, but after actually working through some of it as instructed I enjoyed it and really developed a solid understanding. Simple examples like Vending Machines and Washers really helped me, I still use them in my head when thinking about problems. Design Patterns are not always the best solution and forcing them on problems can make things worse, but as far as teaching Java I really recommend it. http://www.amazon.com/First-Design-Patterns-Elisabeth-Freema...

I also recommend learning about Test Driven Development as previously suggested, we use JUnit4 to teach our undergrads at my university. There are also other methods of testing besides TDD, but I feel it was the easiest to help students.