Hello friends: I have been using a lot of methods to advertise my sites including social bookmarking sites, web directory, article directory, etc. My websites are indexed good for some keywords for some days. But these keywords are not indexed at all for some days. I don't know what causes this. Is there any good SeO tips to get my sites indexed permanently for some keywords? thanks very much!
I think you're nomenclature is a bit off. Sites don't get indexed for keywords. URLs get "indexed"... Then those indexed URLs "rank" for particular keyword phrases. Indexing is not keyword related... A page/URL is either indexed or not. If it's indexed then it either ranks for a particular keyword phrase or not. Getting a URL indexed is quite simple... Getting it to rank well for the keyword phrase you are targeting is much harder... There are both on-page and off-page techniques that you'll want to familiarize yourself with in order to help your page rank.
Are you talking about keywords ranking? That involves much more work to do, among which the quality backlinks are the most important one.
"keep my site indexed permanently for certain keywords" -> Are you sure you have submitted a sitemap via google webmaster tools? That is the best way to keep all your pages indexed, and in those pages add the appropriate keywords and their related content.
Keywords ranking is complicated sometimes but keep it up. It works if you it properly but i´ve yet to find a definitive method.