Well, actually this is pretty simple. Your system should be designed to allow horizontal scalability. And most of the
Java EE applications do. When load increases, you add more application servers to handle the load and load balancing takes care of distribution of the load (if you use session EJBs, you have that functionality out of the box). Withe horizontal scalability, you have to consider all thigs that come with it - for instance distributed transactions (if you're gonna use them), session state synchronization etc.
Another thing is scalability of the database as long as database could be (and it often is) major problem or bottleneck. Here you can consider scalability options of the well known database systems - for instance Oracle has the RAC.
I hope this helps.