When you want to make your application Clusterable, you have to first identify what are thing 
(which holds the State data)should be synchronized among all the Machines in the Cluster for
supporting High avialblity Without impacting the application functionality.
Here are the list of things you should look at.

HTTP Session (Session Replication) :
     In a Clustered ENV, your application server keep the HttpSession state in Sync among all
  the Servers in Cluster, this could be by using different communication technique
  like IP MultiCast etc. So for better performance you have to do the following.

        i) Try to keep little data in HttpSession, do not keep pretty big Object graphs in
  HttpSession, becase each change to Session Data would triggers  session replication
  which serializes the Session state and transport that to all the Servers in cluster.

     ii) All the Objects stored in Session should be Seriazable , if not Session
   replication would be failed.

Stateful session beans :
   You should take the same considetaions as above, even in StateFul Session Beans case.

Singleton classes :
     By design as Singleton classes would be maintained as One Instance per JVM, so making
  a Singleton class Custable is a little Chanllenging as all the JVMs in any given Clusttered
  ENV would have their own Instance of Singleton classes.  So you have to manually keep 
  the  One possible solution for this issue is below.

i)  Serialize the First Instance of a singleton class and store in Database. Serialize the
    First Instance of a singleton class which is created on any JVM and store that in Database,
 let all other JVMs check in Database if a singleton class was already created before
 Creating their own Instance, this way we can make sure that the same Instance is being used
 by all the Servers.

JMS Queues:
You may need to use Distributed Queues or Topics.

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