choosing keywords should be done carefully. when you are doing it manually you better concentrate on the words which are related to your site. the more your keyword relates your site the more you get visitors. effective keywords will help in increasing your site traffic. proper keywords to your site will help your site position improve. for eg. keyword like brainyquote will help in maintaining the site position for a site like
www.brainyquote.com. make sure you enter your keyword in the title or some other place in your page. keep note on your competitors too. the more you get competition, you can more keywords. and of course key word tool too. more than doing it manually if you get assistance you will get more effective keywords. there are many keyword generation tools available now a days and i got it from
TheWebPole.com. they generate very good key word for my site.