Seriously?  I'd hire a UI expert and maybe a graphic designer.

Often, the difference between a home-brewed app and an attractive
commercial offering is the look and usability.  There are people who
have degrees and natural aptitude for this stuff.  Most software
people don't.

I feel your question is a bit missdirected: as far as I can tell, you seem to
think it's a technology issue but I don't think it is. You can develop a
professional looking application using swing, swt, 3rd party GUI libraries
with loads of neat looking controls.
It all about the ergonomics of your program and that's a hard thing to learn
or teach. I'm sorry my comments aren't very concrete or usable, but maybe they
will make you think in another direction.

I provide free articles, presentation, libraries, and demos
that help you understand how to make your Java apps look better.
Besides that some of these libraries help you save time and money
because they streamline your Swing production process.

A good starting point is the "First Aid for Swing" presentation.
See the articles page at http://www.jgoodies.com/articles/
I tried to come up with things I see again and again in  Swing UIs.
And often small things can significantly improve your user interface.
Check if you understand these Don'ts and Do's and if you feel able
to actually implement them.

The presentation includes many screenshots, horrible, poor,
better, good - all which a typical developer can achieve.
You can download and test drive the JGoodies tools and demos
to see what *you* can get with Java and Swing, see here:
http://www.jgoodies.com/freeware/index.html

Often a professional Look&Feel helps or a set of good L&fs.
I provide the free JGoodies Looks that have been optimized
for precise micro design. But many people have reviewed them
as elegant. In the First Aid presentation I mention things
you should look for if you use or purchase a Look&Feel.

The next step is layout and I provide a presentation about that,
a whitepaper, a free layout system, and a free tutorial.
See the articles page and the JGoodies Forms layout system at
http://www.jgoodies.com/freeware/forms/index.html

If you are looking for a good Swing book, consider
Scott Delap's "Desktop Java Live". He addresses the
major Swing problems that every Swing developer faces:
appearance, data binding, data validation, threading,
and other things.

Hope this helps. Kind regards,
Karsten Lentzsch