I recommend getting webstats for any site so one can see who is visiting, where they are coming, what browsers they are using, what links referred them to one's site, and so on. This info is interesting to see but it also lets you to who is talking about your site (through the referral links). The real good ones will show you what keywords people used to get to your site. You can then use this info to pinpoint your own keywords and your internet marketing strategy.
One of the biggest mistakes people make with their websites is they put them up and then they never add any new content - basically the sites remain static. This can hurt you in search engine results because the search engine might think your site is no longer active (aka alive) and they might start to de-list your site in the ratings. This won't be a problem for torstan's or GP's sites since they will be adding new content as they produce/accept it (as the case may be). I've seen too many business sites that do this.
In this case, I recommend one should always keep their sitemap.xml file up to date with the latest changes (make changes to the date on the pages you've changed) because search engines will pick up on this. Google and Yahoo allow you to make an account there and upload the sitemap.xml file directly tyo them to help your site get indexed quicker/kept up to date.
The other nice thing about the robot.txt file (combined with info from webstats) is if you're getting hit by robots from dubious sites, you can ban those robots from accessing your site.
I've only touch the tip of the iceberg about SEO. I'm available for consultations (when I'm not mapping or writing xHTML). That's enough SEO stuff from me. I've got some mapping to do.