Don't use robots.txt to ban large portions of your site

In general, using a robots.txt file to keep certain areas of your site off-limits to spiders can be a good idea. But if you ban significant portions of your site (more than half), search engine spiders may mark your site as "forbidden" in general and simply stop spidering your site as often. And if your site is spidered less often, fewer pages will be added to the directory and updated in rank.