Place the title, tabs, or some other strong element at
the upper left of the page; place the button(s) at the
lower right. Content of any width goes in between. If
the content itself contributes to the balance of the
page, so much the better -- don't put too much white
space on only one side, for instance.
Consider what the color dialog above would look like if
you pushed the OK and Cancel buttons to the left edge
instead of the right edge. The whole dialog would feel
left-weighted and precarious!
In Windows, the placement of the title in the upper left
and the conventional placement of buttons in the lower
right do this for you automatically. In Mac OS X, many
elements tend to be centered -- title bars, tabs, and
action buttons -- so it's much less common there.
Kevin Mullet and Darrell Sano's classic pre-Web book
Designing Visual Interfaces (Sun Microsystems) describes
the idea of diagonal balance on pages 102-104: "Symmetrical layouts
provide ... visual equilibrium automatically. Asymmetrical
layouts can achieve equilibrium as well, but their tenser,
more dramatic form of balance, depends on careful
manipulation to compensate visually for differences in the
size, position, and value of major elements." See below
for examples of how you can achieve this balance.