I'm relatively new to the technical side of SEO and have been trying to brush up my skills by going through Google's online Web-master Academy, which suggests that you need a If-Modified-Since HTTP Header tag on your site. I checked and apparently our web server doesn't support this.
I've been told by a good colleague that the If-Modified-Since tag is no longer relevant as the spiders will frequently revisit a site as long as you regularly update and refresh the content (which we do).
However our site doesn't seem to of been reindexed for a while as the cached version's are still showing the pages from over a month ago.
So two question really - is the If-Modified-Since HTTP Header still relevant and should I make sure this is included?
And is there anything else I should be doing to make sure the spiders crawl our pages? (apart from keeping them nice, fresh and useful)