Have you tried the hairball "treats" you can find in the treat section. You have to be careful, they aren't treats you can give at will -- follow the instructions like you would medicine. Generally you give several at once about every other day. These have worked well for me. (Just remember, don't put out a couple whenever you like as a "treat", follow the instructions on the package). The usually come in a pack that look like regular treats, but have "Hairball Remedy" or something like that written on them.
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Tammy and Pookey
(Stanley 8/91 - 8/07)