@ribonzz Just practice...F2L should be under 15 seconds to be under 20 seconds. It just takes work. http://www.youtube.com/user/lordskull14
That's my account...the vid on that page is what I averaged a few months ago..i'm around 17 seconds now. If you could record a few solves and post the link then I might be able to help you get a bit faster.