For me personally, I would spend the money that you are using on flyers and use it to do more Facebook advertising.
But Facebook offers some free ways to advertise your event as well. You can always create a specific event and invite all of your Facebook friends to get the list started.
You can also join Yardsale groups on Facebook and list your event there. There are groups in pretty much every area of your city - at least that's how it is here.
You could always try and partner with a local scout group as well to help you get the world out on the clothing drive.
Another free option for getting more people to donate is to go create a Reddit account, search for your cities local reddit community and post about it there.