Java Package Names
Always make sure you use a unique package name. If you're not a java programmer, and baulk at the idea of using long names, get used to it. It's really not that big a deal and the rest of the world manages to do so. You'll also find Java IDE's are very convenient at helping you manage them (hardly ever need to write one of these out in full).
com.github.<organisation>.<repo_name> is a good convention for github stacks when you don't have your own unique url.
Maven Group Names
It's also a good idea to use the same fully qualified package name for the maven group name (i.e. the one you insert in build.gradle).
An important exception to the above rules are for unofficial message artifacts. If you want a seamless transition from unofficial to official, or between officially released artifacts and artifacts generated from sources, be sure to set the maven group name as org.ros.rosjava_messages.
Best Practices - some comments from Damon Kohler.