I've been working on search engine optimization for my site SelectiveMobile.com. I'm having trouble finding the right keywords for the keyword tag and don't know how many keywords I should have per page. Please advise.
Depends on the page and what keywords. Without looking at your site here is what I would do... Have a sitewide menu listing all your major keywords and a link to a page dedicated for that keyword using appropriate anchor text, basic SEO, and good unique content.
and remember the fact, that onpage keywords are just one part, more important are offpage keywords, means the keywords used in and around links to your site
To find good keywords for site you should do a bit of keyword research using tools like goodkeywords. You should also spy on your best ranking competitors' sites. That should give you ideas. The ideal number of keywords you should optimze your page for is 1. You should avoid spamming by all means however. This one keyword is the one that you wish to rank high for in the long term. I said 1 is ideal but so what... You can sprinkle some less competitive keywords here and there. I usually like to use ones with zero competition on Google for this. Use the rest of the keywords in articles, blog postings etc
say for e.g. you have a page that sells nokia phones i would have 1 main keyword and 2 or 3 extra keywords for that page. some keyword examples might be main keyword: Buy Nokia Phones extra keywords: Cheap Nokia Phone Nokia Phone Deals Buy Nokia Mobile Phones etc
I like wordtracker's free trial to find keywords: http://www.wordtracker.com/free-trial.html There is no "perfect" keyword density. The best thing to do is to compare the density of the highest ranked websites for the terms you want. I like http://www.seochat.com/seo-tools/keyword-density/ this tool for density