I'm not a developer, but I do tweak/build UIs and refine user processes as a day job and hobby so I think I'd have some good tips here that I've found:

- the shortest distance between two points is the time it takes to get there; in other words, the fewer steps to complete a task, the better the person will feel about your application

- on a mobile device, popup windows should only be used for alert messages; using them as input panels is the wrong user interface model for less than 5in screens

- using a linear user process flow for applications that are composition based (think > write > publish)

- use consistent icons and fonts to the operating system; however, seperate the content from the presentation layer so that the user can do minor, or you the developer can do major changes with least friction to the entire process.

There are probably a few more; Lord knows that I harp on this stuff way too much on my blogs and device reviews. But those are a few easy-to-do-now kind of things that don't need new toolkits to employ.