I've been very successful with garage sales (and estate sales for close family), but I usually have a purpose other than earning the money for myself.
I've run personal sales to earn money for charities, and I've gone around the neighborhoods and grabbed the leftovers from other sales, things other people are getting rid of anyway around the area (give me your stuff please post on craigslist) plus whatever I have in my home...then I have a giant sale. When people want to haggle, I tell them it's for charity...if they want to come back on the last day, then they are welcome. A lot of people understand, and sometimes they just make a donation.
Of course, I have a ton of stuff left, and then I try to find someone else who might need it for their fundraiser, or I itemize and donate to the little thrift store a few blocks away.
Of course, I HATE running these sales. I'm taking a break for a while...possibly forever. And the next time I have to run an estate sale for a family member, I'll probably go crazy.