Referrals are my best advertising.
I also tend to get clients by training my clients outside on nice days. Other people see us training or see me lugging equipment in/out of the car and ask about my services. A good professional business card comes in handy here.
I’ve also placed on-line ads with local moms groups (I specialize in training moms with small children, pre/post-natal). I’ve gotten several clients this way as well.
The great thing about all these suggestions, most of them cost you nothing!