Note - I no longer use MT.
A while ago, google launched their sitemap protocol. A sitemap is an XML file which points at all the pages on the site and helps google to index the site efficiently.
It indicates to google which pages are updated frequently, how important pages are (within the site) and so forth.
You may need to modify it if you're using different types of archive to me. Be aware that the priority tag is only important in the context of your site, you can't improve your ranking by giving things a silly number. Priority tells google that, given two pages with the same matches on your site, which one is more important. [more info]
I have given the homepage a priority of 1, recent articles are pretty important, but I've given the archive pages more importance than old individual entry pages. Note that I have an explicit link to an index on my site which may well not exist on yours!
The relative importance and update frequency are advisory only. Google may well ignore them!
The resulting sitemap is pretty boring to read, but it could reduce spidering traffic.
Once your sitemap is created, you need to submit it to google (it is a shame that there isn't an autodiscovery thing, it could then be picked up by other bots). The sitemap isn't picked up instantly, so you will need to come back to check for errors.
As I write, I haven't put in a 'last modified field' for category or date archives. I aim to do this and when I do the linked template will quietly change.
Your sitemap should rebuild with the site, and a good output name is sitemap.xml
Now... are any of the other search engines using sitemaps?
Further reading: Tips for Blogger Google Sitemaps could work against webmasters Google Sitemaps just got better Google Sitemap for Idiots How to make use of Google Sitemaps Yahoo Submitpage has it's own sitelist: "You can also provide the location of a text file containing a list of URLs, one URL per line, say urllist.txt. We also recognize compressed versions of the file, say urllist.gz"