There are a lot of great suggestions already here, but I thought I would share my own experience. My outdoor cat came in contact with my inside cats, exposing them to fleas. The thing about fleas is it's not as simple as a bath. Baths help eliminate the adult fleas. You then need to check them (lie them on their belly) for fleas and pick off any of the ones you see and flush them (this only works if they have lighter fur). When you're done with this, brush them with a come, which will help get the flea eggs off. You should vacuum the spot you brushed your cats immediately after. You'll then need to apply flea medication to the back of their necks. This will also help get rid of the fleas. If you have multiple cats, however, you need to keep them separate or they'll lick it off each other (this is why you put it on the back of their necks in the first place, so they can't reach). This will dramatically reduce your problem. You should also wash bed linens very regularly, and make sure your cats can't get into the area where you keep your clothes or towels. You should also change your litter box one to two times a day.
A very important thing you need to look out for: FLEAS CAN LEAD TO WORMS. Cats eat fleas all the time, because they're scratching or gnawing on themselves or grooming their fellow cats. When this happens, they are exposed to worms (since the worms live inside the fleas). These worms live in their stomach and cause serious problems for them. It's very common for the cats to develop worms when dealing with fleas. The best way to treat for worms is to do all of the above and also dose them with VET approved medication, which they need to take orally. It's usually a liquid, and they pretty much hate it, but it's important. It will help get rid of the worms. You'll notice odd little segments (short and yellow) around the house after, which means the medication is working. You need to vacuum and do laundry very very regularly as well as do all the steps in the first paragraph.
With any luck, you'll be able to get rid of your problem, but fighting fleas is a very long battle.
