I usually dredge the fish in plain flour first
and then the beer batter.
I actually made beer battered fish tacos for dinner last night and the batter stuck perfectly.
It probably also depends on your batter consistency.
The recipe I use says the batter should be slightly more liquid than pancake batter.
cooking.stackexchange.com/users/61/Bryant
Give your fish a light dusting of corn starch before dredging in the batter.
Flour works to some extent but without gluten formation flour is working as a drying agent more than anything else.
Corn starch is, as its name implies, a starch which will actually act as a weak glue when heated wet.
Just don't pile it on.
dredge each fillet in the corn starch and tap off the excess until you're left with a thin, uniform layer.
Your batter will stick to the corn starch, which will stick to the fish.
cooking.stackexchange.com/users/990/Mike Yockey
If you did AB's recipe , you need to lightly dredge the fish in cornstarch as @yock mentioned.
You should also make sure that the batter is cold -refrigerate for 15 minutes to an hour at most.
It turns out pretty good if you follow that recipe.
cooking.stackexchange.com/users/207/Recep
If it's English style beer battered fish
the batter "shouldn't" stick to the fish
otherwise it will be too dry or soggy depending on which way you go rather than light and crisp.
To be light and crisp it should not adhere to the surface of the fish too closely.
Breaded fish is a different matter.
Take a look at the pic here: http://www.bbcgoodfood.com/recipes/7785/golden-beerbattered-fish-with-chips
cooking.stackexchange.com/users/486/tonylo
Pretty sure that the problem you are having is moisture; here's how I do it.
Take the fillets, pack them with flour and set them on a wire trivet and let them dry a bit and then shake them off then let them dry some more.
When the outside of the fish is dry to the touch I dip them carefully in the batter and cook them.
This method seals the fish inside the batter 'envelope' holding in the juices for that first delicious bite.
Note that if the batter is too thick the outside of the batter will contract so much faster than the inside that it will crack, allowing fat in and letting the juices out.
While the last of the fish is cooking I will add an egg, some cornmeal and spices etc to the left over batter and make hush puppies in the same pan I cooked the fish in.
cooking.stackexchange.com/users/7092/Frankie
Be sure to pat the fish dry as completely as you can.
Don't salt the fish beforehand to avoid drawing out any moisture.
Also, like Frankie suggested, dip the fish slowly into the oil.
Dipping it slowly allows some of the initial moisture to escape and lets the batter cling to the fish before there's a shell of batter holding in all the moisture.
cooking.stackexchange.com/users/21198/Rosa
A few months back I made beer-batter fish.  I don't remember the exact recipe I used but I think I got it from foodnetwork.com.  I fried in vegetable oil at 350.  Everything came out OK, but the breading seemed to peel clean off of the fish while eating rather than sticking to it.  Are there any tricks for prep/cook to prevent this from happening?
Beer batter fish, keeping the breading from peeling right off while eating?
cooking.stackexchange.com
<fish><deep-frying><batter><breading>
cooking.stackexchange.com/questions/954
cooking.stackexchange.com/users/14/shsteimer
Don't salt the fish beforehand to avoid drawing out moisture, use a batter that is slightly more liquid than pancake batter and dust the fish with flour or corn starch before dredging in the batter. You should coat your fish in flour or corn starch before soaking in batter. Your batter should be slightly more liquid than pancake abbter.
Soak your fish in flour or corn starch first. The batter should be slightly more liquid that pancake batter. Don't salt the fish as this will draw out moisture.