The only way to make this taste right is to use homemade chicken stock and shrimp stock. And to make it the day before you're going to serve it so that all the flavors can mature - it always tastes better the next day.
I've been told by some of my friends, who grew up in New Orleans, that this is pretty authentic.
While that's going on, put your chicken, shrimp, and crab into a large bowl, add 4 T of your cajun spices, and toss to make sure everything gets covered nicely.
Once your onions are translucent, add the chicken, shrimp, and crab to the pot, and stir. After about 10 minutes or so, add your peppers, open a beer (preferably an Abita), and continue to stir the pot.
After about 5 more minutes, add your chicken stock and shrimp stock and your bay leaf and stir. When the pot starts to boil, reduce your heat to allow it to simmer, and add your tomatoes. When it returns to a simmer, add the rest of your cajun spices and filé powder and stir.
After about another 15 - 20 minutes, your gumbo will be "done" cooking. That said, the flavors and spices want to mature, so you can either drop the flame down to low and allow it to continue simmering for an hour, or you can kill the heat entirely, let it cool, put it in the fridge over night, and re-heat it the next day.
Serve in bowls, over rice, with a dash of filé powder sprinkled over the top.