Optimizing Old Posts
It doesn’t hurt changing old stuff, but it likely won’t help you much. Time would be better spent by building links or creating a new page of your best photos and marketing text.
Google doesn’t use the “keywords” meta tag in its web search ranking.
Link exchanges, paid links, and forum comments with irrelevant keywords linked from the signature all violate Google’s quality guidelines.
Listing a bunch of keywords at the end of a page violates Google’s quality guidelines.
Using the same phrase at the end of every title or image won’t help you rank higher and is an opportunity missed to rank for the hundreds of other possible keywords you could have used instead.
Hidden Text, Cloaking
Anything you hide from users, Google will hide in search. I believe this extends to HTML shadow sites too.
Buying keyword-rich domain names and forwarding them is a web spam tactic that doesn’t help SEO (because the site has no text or inbound links).
Flash sites don’t have text on the page or alt text for images, which makes it nearly impossible for a search engine to read the page. Even if the Flash site has an HTML shadow, Google still won’t want to rank it because that text is hidden from users.
Commenting on Other Blogs or in Forums
Google doesn’t give value to comments or forum signatures, so use discussions to discuss and not for SEO.