Oh, this is my specialty.
I'm a pet groomer, here are some tricks of the trade that I've picked up over the years:
Use a flea collar, not on the cats, but put it in the vacuum to help kill the ones that get sucked up.
If you do wash your cats, use plain original dawn dish soap and let soak for 10 minutes. It will work better than any chemical filled flea shampoo. Fleas create a waxy/oily coating to keep from drowning when they detect water, dawn breaks this down and drowns them.
Small items (bedding, couch cushions, etc.) that can't be laundered or vacuumed very well can be placed in a car, park in the hot sun for a few hours to kill any buggies.
You can make a trap for fleas, put a little water (enough to drown them) in a pie pan, add some dawn dish soap, mix it up but gently so there are no suds. Place the pan under a lamp on the floor, it is supposed to attract and drown them at night.
Treat the yard, many people skip this step but if the yard is infested they will keep on coming right back in.
Treat areas you might not think of such as windowsills, etc. where the cats might hang out.
Look into food grade (it also comes in a pool grade) diamatacous earth (possibly misspelled). You can safely put it on the pets themselves, carpeting, bedding, etc. I don't know what stores sell it, but some online pet supply companies do carry it.