Brew a strong concentrated tea using herbs like rosemary, neem leaf*, and lavender. Can also cook a lemon half in water for 20 mins, strain and add to the tea mix. Apply on kitty and leave on. If your cat gets too enthusiastic grooming him/herself following application, help things along by wiping with a microfiber cloth or hand towel (dipped in warm water then wrung) to remove some of the herbal liquid.
1/4-1/3 cup comprised of a mix of:
1 part lavender or rose geranium (also works on ticks) hydrolat
1 part lemon verbena hydrolat (can substitute with rosemary or citronella hydrosol/hydrolat)
2 parts water
Get a commercial flea trap, or make your own by adding a few drops of dishwashing liquid to water in a pie plate or very shallow bowl. Place under a small lamp or light source. In the morning, you'll see dead fleas in there. Dump the contents and start over. Kitties don't usually bother with these traps but as always if yours are different, you'll have to be creative as to where to place these traps.
Make sachets at home containing dried eucalyptus leaf, neem leaf, cedar pieces (not shavings), pennyroyal, and lavender. Or place 1 teaspoon of this mix in any cloth square and tie using a rubber band or ribbon. Place these in corners, tucked into furniture, and other areas around the house in places where your cats won't be messing with them. Mine don't, and they're quite mischievous as a general thing but YMMV. You can always add some dried citrus rind/peel in the sachet/handkerchief to ensure kitties don't go near it.
Place salt in small plates/saucers in as many rooms as possible. Remove and replace periodically.
Place cedar blocks in various area. If you use small cedar balls used to repel moths, they can become cat toys so blocks are safer. As long as the cedar isn't in shavings form the aromatic oils won't bother cats, so placing a cedar block under their bedding or in areas they hang out shouldn't harm them.
Put some of these dried herbs in the vacuum bag to kill fleas that get sucked up during the vacuuming process. Later, remember to dispose of the bag promptly, or if you have a filter-less vacuum, empty the contents outside and wipe down the vacuum prior to bringing it back into the house.
* Vacuuming kills fleas. Researchers at Ohio State University found that vacuuming kills not just adult fleas, but also pupae and larvae: