I have been doing some experiments on my gadget review blog lately (which is run on WordPress) and here’s what I discovered:

1) Never allow search engines to index your tag/archive/category pages. My rankings dropped dramatically since I allowed that and from 2k unique visits from Big G I’m now receiving barely 200.

2) Do not use the following permalink structure: blogurl.com/category/postname/; I used it and my rankings fell even more.

3) Enable “Show full post” in wp options, don’t make it show a brief of the article then a “Read the full article” link. Search engines sometimes treat is as duplicate content

4) Do NOT create tags. Even if you make search engines disallow them, if one was already indexed, it’s too late. NEVER create tag pages, I will always remember this mistake. It still costs me lots of visitors.

That’s about all I discovered so far. :)