This week's book giveaway is in the OCAJP forum. We're giving away four copies of Programmer's Guide to Java SE 8 Oracle Certified Associate (OCA) and have Khalid A Mughal & Rolf W Rasmussen on-line! See this thread for details.
Your question is really vague but I suggest that you check the scopes you are using. Maybe you are storing a lot of things in the session-scope (for example)?
What about profiling your application? VisualVM, jProfiler and YourKit are all useful and will shed a lot of lights on the dark corners.