Button/control design recommendations
Place the most frequently accessed controls near the bottom of the form. The user interacts most often with the output area, with the icons in the output area, and with the hard keys on the system. Placing controls at the bottom of the output area/form puts them as close as possible to the output area, making them quicker and easier to access.
People from Western cultures tend to read the screen from top to bottom and left to right. Therefore, anything important for users to read (rather than interact with) should be near the top of the output area/form. Horizontally, you should arrange controls so that the leftmost control is the most important.
Do not clutter the screen. Running out of space is a usually a sign that simplification is needed. Squashing a lot of controls on the form by reducing white space is usually the wrong answer.
In most cases, use spacing instead of lines and boxes to separate user interface elements into logical groups. Lines and boxes add to screen clutter and actually make a small screen harder to read.
Provide a label for any control or option that requires further explanation. Right justify the labels and left justify the fields. Use bold font and title capitalization for labels. That is, capitalize the first letter of each important word in the label in the same way you would capitalize the title of a book. For example, use "Set Date" as a label, not "Set date." Never use all lowercase ("set date") or all uppercase ("SET DATE") for labels.
[updated from http://www.palmos.com/dev/support/docs/ui/UIGuide_Front.html]