Many answers have been given and many of them are illegal so be sure to check where you put your flyer before posting it.
I will give my 2 favorite places where I have had the best success.
1) Grocery store bulletin boards
2) Colleges
These 2 places are great because everyone needs food and young people are the future of SFI.
Grocery Store - Every grocery store I've been in has a free bulletin board. Hundreds will see your flyer everyday and you can check on your flyer every time you visit or post a new one if it is gone. It is a very easy and beneficial way to enhance your SFI business.
College - Young people are great with the Internet and have a lot of contacts via social media such as Facebook. They are the future of SFI so get them now before someone else does!