Do you have a link to the site in question?
Typically, the more new (and high-quality, quality, not quantity) is useful.
Write for people, not for bots. Too many keywords will penalize you.
Setup good 404 pages, with links.
Robots don't usually care about design if it is applied through css, but if it's purely html styling (tags and tables only), it might be a good idea to switch to css.
Do you have a link to the site in question?

I guess with a continuous link building then you can update your content when your ranks started to going down. So that it will not sunk more lower. Then customized your 404 pages so that your visitors won't leave your site so fast because they will be entertained by the 404 page you created.

Proper analysis after every two months can help you a lot, keeping track on keyword ranking, directory submission to relevant niche and try to submit your articles in some other article directories too and take a look about the time span they are taking for your article approval.

Here are somethings to avoid when creating your website:

1. Avoid strange color combinations, as this can make it hard to read for a viewer
2. Text that is too small to read will also cause the reader to leave
3. Unrelated Advertisements will distract viewers
4. Hidden text, which is text the same color as the background
5. Submitting your site to FFA sites
6. Duplicate pages, doorway pages and cloaked pages
7. Re-direction from the homepage to another
8. Automated submission to search engines
9. Submitting to search engines more than once a month
10. Use of <meta refresh> tag in HTML page header
11. Using trademarks or company names other than yours in Meta tags.
12. Instance of 5 or 6 keywords being repeated in one row