I think that the best "cheatcode" is "Don't be smart, write dumb code".
The terms are somewhat loaded but basically you want things to be simple when you write them, because then they will be simple to read when you come back to it later. Also these days the JVM is a LOT smarter than you are, and non-dumb code usually try to do something "more efficiently" which may prohibit the JVM from doing something even smarter.
Have a look at http://java.sun.com/developer/technicalArticles/Interviews/devinsight_1/
Please note: It is ok to write smart code, you just need to prove first it is necessary :)
EDIT: A few more things.
Write a lot of code. Practice makes perfect - always strive to do this in the best way possible. If you come back to your code later, you might have learned a thing or two since which allow you to perspective your earlier choices allowing you to make a better descision next time you have to make a choice. THIS INCLUDES DOCUMENTATION! Documentation is what allow others to use your code without understanding it in detail.
Read a lot of code. Set up your IDE so it allows you to see as much source as possible for what you work with. By using a JDK as the JVM in Eclipse the sources are automatically attached so you can SEE the source for whatever you happen to hit in a breakpoint. When including a jar from e.g. Apache in your code, include the sources so you can see what it does. It will help you some day that you can figure out WHY a certain thing happens and how to avoid it.
Work with peers. Other programmers on a similar level may provide input to a situiation - perhaps even just their listening to your explanation clarifies things in your mind - and you may also help them. By working together and evaluating together you can leverage each other.
The Java Tutorial from Sun covers a LOT of the Java libraries, and learning those covering the essential part of the runtime (java.lang, java.io) by heart and having read the rest (just to know what exists) will make you a more skilled programmer. You will know what is in your toolbox!
Participate in Stack Overflow. Writing good answers to questions (even if they have been answered but badly) will allow you to put your experiences into words, which is practice for writing documentation :)
