Just two things, really:
- "A user interface is well-designed when the program behaves exactly how the user thought it would" - quoted from Joel Spolsky's User Interface Design For Programmers
- Put your designs in front of a user. A real end-user is best, but for lightweight, rapid feedback, you can't beat hallway usability testing i.e. grab a co-worker.
If you remember Joel's advice and make sure you get feedback on whatever you do and act on it i.e. iterate, you'll not go too far wrong. And I would echo the recommendation for Steve Krug's Don't Make Me Think - it's probably the best work-related book I've read, bar none, and is just as applicable to desktop software as websites.
Hope this helps.
