JesseLY is correct, its the biggest problem in website creation, getting the content. Once you do get it its either to much or two little, but in this case more is always better I reckon, at least i can be trimmed down.
One format you could investigate is the complete reversal of starting the site before hand without the content. Stipulate that until all required content has been received and approved then and only then will you begin the actually process of designing and building the site.
This of course may turn some heads and have its own disadvantages but the major advantage is that all the parts of the puzzle will be in place and the design of the site can only improve from there as a full overview is available. Also this will minimise the client delays to a much smaller percentage.
Does anyone actually follow this practice and employ it, are the results positive or negative?