Well, there are two solutions, one quite simple, the other less so.
You can design at 800x600 by default and accept that higher monitor resolutions will have a band of unused space on each page, or you can assign percentage values to your page layouts.
For a very crude example, let's say you create a new site page.
Now let's say you put a single cell table on the page, but you assign a table width of 90% instead of a pixel width. This means that the table and anything in it will always occupy 90% or the available screen width, regardless of resolution.
Now let's say you add another table inside the first one with three rows and two columns, but set it's width at 100%.
You have a main table that will always be 90% of the browser window with a layout table inside it that will always be 100% of the main table width (90% of the avialable screen width). It sounds far more compex than it actually is.
Merge the cells in the top row of the second table you added, then merge the cells in the bottom row. You now have a header space for a logo or banner, a footer space for your copyright statement, and a main content window area divided into two. Give the to and bottom cells of the layout table height dimensions in pixels if you like.
Now let's say you make the left cell of the main content window 120 pixels wide by giving this value to the cell in the cell properties. This is usually about wide enough for standard width navigation buttons, hyperlinks and what have you. Your right cell will contain the main page content and since you have a 100% table width combined with a specified pixel width of the left navigation button cell, the content cell should resize on the fly depending on the screen resolution it is viewed at.
As I say, this is a very crude example and not ideal by any stretch of the imagination, but it works and should get you thinking about your layouts in terms of percentages instead of your own computers native resolution.
Percentage layouts are fluid and cater to the majority of resolutions, but they have some issues that may need to be dealt with.
Before we go into the possible downsides though, just try an experiment based on what I've said above and see where you get.