The feather duster may be putting a static charge on the keyboard and making it worse.

If you want to spend the money, they make tiny vaccuum cleaners specifically for getting in between the keys of keyboards.

I usually just vaccuum mine wholesale with a brush attachment of my regular vac. Doesn't pick up all between the keys though.

Can of compressed air to blow the dust out is a great idea too.

Finally maybe get a cover for the keyboard for when you are not using it to keep extra dust out. Does the cat like to sleep on the keyboard? A cover will go a long way if you use it consistantly.