How do other sitters keep their home flea free with so many dogs coming in and out?

Even for treated dogs, there's always the chance of a flea hitching a ride into your home. Although the flea will not live on the dog, the flea or eggs could end up in the house. It may help to keep checking and brushing all dogs after being outside.

With carpet, you've got an additional challenge for sure. If you spot a problem inside, every rug, linen, etc. must be washed in hot water in the washer, the floors must be cleaned, and when the carpet is vacuumed - immediately after empty/toss the vacuum filter outside.

There have been some other posts that you may find helpful by searching fleas.

Deb is correct, fleas are tricky! You're definitely doing the right thing by asking owners to do their part in ensuring their pets are up to date on flea preventative.

I block off 2 days at the beginning of each month where I treat my house for fleas. Since I live in a town home, I arrange for pest control to come on the 3rd of each month to treat our perimeter for mosquitoes (heart worms are a big problem in our area) and fleas.

For inside the home, I flea bomb every two weeks. It takes several hours. So I try to arrange to have my dogs (and any guest dogs) at the dog park during that time. I offer free baths for guest dogs and this gives me the chance to manually check each dog for adult fleas before they leave.

I require up to date flea and tick prevention for boarders; my intake paperwork has a spot for the brand of flea/tick prevention and date of last dose.
I do not accept flea/tick collars as an adequate form of prevention. I worked at a spay/neuter clinic for four and a half years, and if I saw one on a dog, I could almost guarantee you the dog had fleas. I found a flea on a boarder several years ago, and immediately contacted the owner (a friend). I was able to dose him with a Capstar from work, and we also did a flea bath.