I have found adding some Iams hairball formula to their food helps, in tough cases I go straight hairball. Hasn't worked everytime but enough to convince me to try that first.
The breeders i bought my cats from both use IAMS hairball. I wonder if thats the reason..that they have found that it helps make firm poops. i dont know if im just going to the wrong store, but i cannot find the hairball formula in the large bags. i had thought about using IAMS hairball at one time because my local mentor/breeder so strongly recommends it. the regular IAMS i can buy in the large bags. It isnt much cheaper than what i feed now though. I feed natural balance ultra premium. costs 30.00 for a 15# bag. I go through 2 a month. Then i have my Newfoundland and German Shepherd to feed, costing me another 60.00 a month. so i totally know how it is having lots of furkids to feed and trying to save money. Im spending 120.00 a month on pet food!