We have garage sales every year. Usually at my Moms house and we usually have about 8-10 different families in the sale - so, it is pretty big. Which is great because more people stop when they see that the garage and driveway is overflowing with stuff. (Many times I will drive by a sale when I only see a few tables set up)
Someone had previously said to "mark everything". I second that. I hate asking how much something is.
Also, I think advertising and signage can be huge. We always advertise in a few papers (we split the cost with all the families in the sale). We also put up tons of signs. Make sure the signs are big enough so people can actually read them when driving by (I hate those little bitty signs that have so much written on it - the only way you could see it is if you stopped your car and got out to look). Also - put arrows to direct traffic to your sale. We also have those flags we hang up in the yard -so, people can see where the sale is. (You know those flags you see in front of new homes and such)
I also hang up as many pieces of clothing as possible. I also group them according to gender and size ( I actually went online and found size markers like in the stores - so, I have 2 of each from 3Months all the way to adult size 18 and XS to XL. These have come in very handy and people can go to the size there are looking for) I sell a lot of kids clothes and I don't mark them to give them away either. But, I also will not sell anything with stains or holes in them.
We also group things in like departments and on separate tables (our church has a a bunch of tables that are older that we can borrow at any time) So, like a table or two each for clothes, toys, holiday stuff, glassware, kitchen stuff, linens, purses/bags etc.
As stated in other threads. I have done goody bags for boys and girls with all the little kids stuff that your kids accumulate from Happy Meals and Birthday Parties and such. I put 5-6 items in each bag and mark them for $1. I sold all 20 of them I made last year.
My work also has a big popcorn machine, so I made popcorn and sold bags of that for $1. Will do that again this year as I did pretty good with that last year.
Can you tell I love having garage sales. I am in one at my friends house in a month. It is the annual Woodbury Sales and it is huge. People come from hours away to shop at this yearly sale. The streets are just car to car with people. I can't wait - it is going to be so much fun (and hopefully make me some good vacation money...)