Many of us have had problems with Google sending 50% or more of our pages to Supplementals. My own situation has improved a great deal, but it’s far from perfect. I’ve tried an SEO plugin I found, but it hasn’t helped as much as I’d have liked.
One of the the changes I have tried is creating original archives pages with an excerpt tool that allows you to configure the length, break points, etc.. of the excerpts that show up. Take a look at any of my categories, and you’ll see the first 120 words posted.
This is a big improvement over the first archives as the full post was made available there. There are still some kinks to be worked out: like how to show more than 3 posts (set by the options in “Reading”), what can I do with my Google Ads, and how long is long enough so that Google doesn’t treat the page as a mere copy of content elsewhere.
You can find the plugin at http://guff.szub.net/2005/02/26/the-excerpt-reloaded/ but the plugin looks a little dated now. Still, I’m finding it working well in Version 2.2 of WP. You should, too.