Google keywords changes every few hours, is there any wordpress plugin that can get those keywords in few hours and put the relevant ones into my posts automatically?
Buddy, stick to a particular Niche and write ur Blog about that niche ... Make sure U add keywords on the bottom.. as far as wordpress is concerned, keyword Ninja is one which can help U gather words of ur competitors..
I agree with haraa, another advice is to write the keywords in bold. This makes them more visible to the search engines. As for the constant updates, you can try pinging your site every day. The content should change as well though, otherwise this is not advisable.
You have to do it yourself, but then that would really devalue your blog and it would just become something like a newspaper instead of something with a value. Stick to a particular niche and focus on that.