What you need, and what you mean when you say focus, depends on what your long-term goals are for your dog.  There’s dog-park focus, rally focus, and intense formal obedience focus.  They are not all the same thing and you work differently for each.  Here’s some pointers for you – take what you can use based on your goals and store the rest for future use if your plans change.

Dog park focus is a dog that is responsible for keeping track of his human.  He checks in regularly to make sure he knows where you are, and comes back willingly when you ask him to.  This behaviour pattern is best started when your dog is a wee puppy.  At that stage, they are still feeling dependant and want to stay close to you.  Playing hide-and-seek is my favourite tool here.  When your puppy is awake, but not watching you, hide around the corner.  Wait a minute until he notices that you’re gone, then call him.  Make a big silly fuss about how clever he is, give him tasty treats, play tug, do whatever your little fuzz ball thinks is really, really fun.  Take it outside and do the same thing (generally in a fenced back yard so he doesn’t get scared when you’re not visible and take off in the wrong direction looking for you).