Default Methods
I know the question said Java 6, but in Java 8 we'll have a pretty decent alternative: default methods.
We'll be able to add 'default' implementations of interface methods, so we can add new methods without breaking every class that implements the interface.
As long as your mixin doesn't need state, you can write code in an interface. Then your class can implement as many of these interfaces as it wants and boom, you've got mixins.
Is this an abuse of the system? A little bit, but it doesn't get into any multiple inheritance issues because there's no state.
Of course, that's also the biggest disadvantage with this approach.
