Submit buttons as a whole are very boring however they are commonly recognized by the Internet community at large while is why many developers leave them to their default settings.
Where a design requirement is needed you may choose to replace the standard Submit button with an image.
Below is code for replacing the Submit button with a Magnifying Glass:
background: transparent url(../images/common/search-magnifying-glass.gif) no-repeat left top;
cursor: pointer; /* hand-shaped cursor */
cursor: hand; /* for IE 5.x */
Make sure te width and the height match the size the the image.