There are a number of techniques that you should avoid.
They are very likely to be flagged by the search engines as spam.
Do not use link farms
Because of the importance in links in search engine rankings,
some people attempted to beat the system by using what's called
"link farms." In short, a link farm is a page with many links and
no real content and no central theme, with the only purpose to increase the number of
links to the destination site. Search engines have gotten a hold
of this technique, and sites associated with link farms can
be banned from the search engine index.
Do not use frame
Frames, especially if nested, are difficult for spiders to
crawl through. As a result, some of your pages may not get indexed.
Do not spam keywords
There are two most obvious ways: Repeating the keywords many times
on a web page, and making text color as the same as the background
color so the users cannot see it. The second way is to display keywords
in really small fonts so that visitors won't really notice them, but
a search engine spider would. Search engines have long ago grown
smart to these techniques, and doing so will get your site banned.
Avoid using redirects
Sometimes, people would set up a page to designed specifically
to focus on particular keywords, and when visitors click on the
link for that page, instead of seeing that page, they are redirected
to a different page. To the search engines, this is considered
spam, as the visitors are not seeing what they intended to see.
Avoid malicious cloaking
Cloaking is the technique of serving up one page to the spiders, and then serving up a different page when a
human visitor shows. There used to be the conception that any type of cloaking is bad. This is not true, as
search engines themselves (such as Google) are employing cloaking techniques. There are legitimate reasons
why a site may want to serve up a version of the web site to the search engines, and a different version to
human visitors. As long as the visible texts are the same, the search engines will not penalize you for it.
Malicious cloaking is for cases where the webmaster serves a page with a very different content than what the
search engine spiders see. If such techniques are caught, the search engines will categorize your site as
spam.