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Initializing the object via a static initializer.
Using the final keyword  on a field to ensure that other threads see a fully constructed object after the constructor returns. The value of the field must be effectively immutable or thread safe. Use Collections.unmodifiable and the like to ensure immutability.
Using the volatile keyword to ensure that threads read the most up to date value. The volatile keyword can be tricky, but is very cheap when update updates are rare.
Guarding the object via another memory barrier like synchronized or a Lock.

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We should try and use concurrency strategies that are efficient. For example, see ConcurrentHashMap, ConcurrentHashMap#newKeySet. We should also be careful to size and configure these data structures appropriately. Using a default sized ConcurrentHashMap for a large set of objects is expensive and generally a waste. Forgetting to set the concurrency level appropriately when there is, for example, classloading level concurrency going on, is also a real problem.

We should not do dumb polling - usually this is laziness. Prefer callbacks and #wait #notify #notifyAll or other such efficient mechanisms.

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