This is probably because you are doing wrong things from the very beginning: fixed sizes or positions of the element, inflexible design.
Please read some material on the problem of nice flexible Web page design:
http://www.flyte.biz/resources/newsletters/04/12-fluid-v-fixed-web-pages.php[
^],
http://www.sitepoint.com/liquid-design/[
^],
http://www.maxdesign.com.au/articles/liquid/[
^],
http://www.digital-web.com/articles/liquid_web_design/[
^].
Find some more:
http://bit.ly/LvyOyr[
^].
Think on the design. Focus on being clear and informative, don't allow yourself to put a single non-functional "decorative" detail. Try to keep it as simple as possible, but not simpler. Make the design not vulnerable not just to the size of the Browser window, but also to those peculiarities of different layout engines; allow your site looking slightly different while remaining recognizable, accessible and well readable.
—SA