Get a notebook and write something in it every day, and be honest about it. Random little things that reminded you of him, thoughts you're mulling over in your head wondering if you did them right or not, conversations you had about him with the people around you; things like that. Then read it back every month, or whenever you feel like he's slipping away in your memory.
But the most important thing you can do is to let him live on in your actions for as long as you're alive. Remember all those things your dog did that made you happy? Now it's your turn to make life better for the people you love, by carrying on doing those things in his stead and not letting that goodness disappear from the world.