Here are a few simple yet important tips when building a mobile widget:

• Auto-focus input fields wherever logical. It’s much easier than using the joystick on the phone.
• Hover effects are important for usability. Make buttons obvious, because getting to them is tough.
• Make sure the phone has your typeface and renders it nicely. Italics might not be rendered natively.
• Don’t resize images with HTML. The phone does it wrong, and it looks horrible.
• Make sure you have a good icon that looks great in Widget Manager.
• Use CSS sprites because it makes it easier to port your widget to the web.
• There’s no keypress-type event handling, so plan on polling if you’re building a game.
• Remember, the phone is fast, but not as fast as your computer is. Neither is the network connection, so be thrifty.

Thanks to Elliott for the help on these tips.