This isn't an actual code tip, but I found a Swing GUI designer for eclipse that's open source, and is actually quite promising. I believe this project is still active because there have been updates to the website recently, but the latest build was in May of 2009.
Even still, the only issue I've been able to find after preliminary testing is that the refresh of the code isn't done "automatically" (I think you have to save the code before GUI updates are made in the code, and I haven't figured how to get code updates to show up in the GUI, if it is possible). There's also not really any documentation that I can find, but I think for the most part the design is fairly straight forward and easy to use.
Project webpage: Visual Swing 4 Eclipse
Note: I don't know why, but I couldn't get the update site to work with Eclipse. However, you can install this plugin by downloading the latest zip from the downloads tab, and then put all the jar files into Eclipse's plugin folder.
A basic intro guide (what i've been able to figure out):
To create a class that can be updated by the plugin, go to the "create new" menu, and go down to Visual Swing Class folder. Choose the type of class you want.
To add other components to this list, go to Window > Show View > Other
then choose Visual Swing > Palette
This will show the palette for adding new components. The panel has most (I think all, but I'm not sure) Swing components. There's also a section for adding custom swing components. I have yet to test this feature out, but I have a feeling this feature might work.
edit: I tested out custom components, and I don't think there's code to actually allow you add your own components to the list, or at least I can't find any.