Many Web sites such as the following provide guidelines for designing UIs:
AskTog — Essays on good design and a list of
First Principles for good user interface design. The author, Bruce
Tognazzini, is a well-respected user interface designer.
GUI Design Handbook — A detailed guide to the
use of UI controls.
Usability Glossary — An extensive glossary
of terms related to UI design, usability, and related topics.
UsabilityNet — Covers design principles, user-centered
design, and other usability and design-related topics.