Required: My Custom CSS plugin
To do this, you will need to add some CSS in the My Custom CSS menu. The primary sidebar (typically on the right) is enclosed in the #sidebar selector, and the secondary sidebar (typically on the left) is enclosed in the #sidebar-alt selector. The main content is enclosed in the #content selector, and the content + the primary sidebar are enclosed in the #content-sidebar-wrap selector. If you are using just one sidebar and adjust the width of #sidebar, you will have to adjust the width of #content by the same number of pixels to compensate.
If you are using two sidebars and adjust the width of #sidebar-alt, you will have to adjust the width of #content-sidebar-wrap by the same number of pixels to compensate. And if you adjust the width of #content-sidebar-wrap, you will also need to adjust the widths of #content and/or #sidebar to compensate. #content-sidebar-wrap usually needs to have an extra 20px than the sum of the pixels from #content and #sidebar.
See the example below.
#content-sidebar-wrap { width: 760px; }
#content { width: 470px; }
#sidebar { width: 270px; }
#sidebar-alt { width: 140px; }
