No matter how good your product or service is, it has been seen that the sites which rank well in these 3 giants are very successful in their business. But SEO for these three search engines might sound challenging, they have totally different algorithm, structure, and three different set of rules. Then how do some sites still manage to be on top of all these three search engines?

The most important factors of SEO:

ContentContent is probably the most important factor, and it solely can determine a lot of your success. Yahoo and Google panelize sites with less and unrelated contents. If you run an automobile site, naturally the search engine presumes that the content inside will be related to automobile. But if from no where you start writing about home loans and debt collection then you might be penalized. Your content doesn't' need to have the same keywords to be considered related. Nowadays the engines are getting much better at determining themes of sites and figuring out the relationship between keywords.

SandboxFor those of you that are not familiar with this term, the sandbox is a penalty imposed on the new sites and new links by the big search engines under the assumption that they cannot play nicely with others. This penalty is strictly followed by Google and to a lesser degree by Yahoo.

Back linksLinks plays an important role to push your site to the top. Links from site with good page rank will certainly help. It has been seen that link higher up on the page holds more value than a link which is at the bottom. Also the age of the link is also important; the older the better.

Density of Keywordsyou should keep this at a very optimal level. Don't try to make your content too keyword specific. There are many third party tools such as IBP, Total optimizer which can calculate the keyword density of your site.
Site Structurethis is particularly important for MSN as they require a site to be structured well with a distinct theme throughout and they also require the site to have some inbound links. It is fairly easy to optimize a site for MSN because it tends to pickup new content and inbound links pretty quickly. A good structured site will refer to a page which can be easily spidered by the search engines. Try to place the content area as high as possible in your html document; minimize the use of formatting tags. Having a content area that starts at line 143 in the code is not the best thing when it comes to SEO.

So if you can look into all the above factors and act accordingly, you are expected to see some good result soon. However it takes time to get to the top of all three giants. Msn takes the least time to index your site provided your site is optimized well.