If you're logging for debugging purposes I suggest setting your application up so that you can change verbosity on the fly. Combine that with a good naming convention within log4j and you could increase the detail in an area of interest while filtering out the rest.
Some parts of your application might not need constant logging. Put the logging code in there anyway and use filtering to keep it from ending up in the log. If you ever run into problems just remove the filter.
If you are communicating with other systems figure out how to tap into that conversation. They might work fine now but sooner or later they'll break.
In the root of your application put a document labeled "HOW TO READ THE LOGS FOR TROUBLESHOOTING". Make sure the filename stands out from all the other files in there. Explain where and how to do the things mentioned above. It might be someone else than you that has to do the troubleshooting. Or you simply forgot how to.