Spiders are great for getting rid of other bugs and insects in your home yet they have a terrible undeserved reputation. In fact, if you truly knew how good they are to have around and didn't have a few as tenants you'd probably let some loose to ward off other unwanted pests.
However, too many spiders can also be nuisance and preventing spider infestations is like preventing any other insect infestation.
Keep the house clean: bugs like trash and yuck and rotten food. You would too if you were a bug. That's why keeping the house tidy is important and even if you think its clean enough ask yourself: where do you put the trash? If your trash cans are near the home it's like practically inviting the insect world to come and join your humble abode.
Bugs will gladly be attracted to the trash, be fruitful and multiply and then you'll have an even bigger situation to deal with. When predator bugs like spiders know they have a constant food supply they'll also set up shop nearby and eventually find a way into your home.
If you run into such a situation where spiders are rampant the most natural way to take care of them besides stepping on them is using diatomaceous earth, a natural non-toxic insecticide that is generally harmless to people but deadly for insects as it disintegrates their exoskeleton killing them.
Place the diatomaceous earth in areas where you know spiders pass including outside. Just be mindful that it will kill any insects that come in contact with it so use the substance sparingly.
Also, its important to remember that like boric acid, diatomaceous earth comes from the shells of microscopic single celled algae and although its harmless to people it may irritate skin and it dangerous if inhaled. Keep out of reach of children and pets and when handling, use a face mask and gloves.