I had a huge 2-day one when I sold my house and got rid of 37 years of stuff I had been dragging around from state to state.
A friend helped me and we had a really good time, most of the time. Met a bunch of neighbors I had never met in the 14 years I lived there. One guy bought almost all my furniture.
I did have to threaten to call the police on this one group of ladies who were there to mostly steal. I don't think they got away with anything important, though.
I'll never have another one. I have nothing left to sell.
My advice:
Have plenty of small bills. The first 10 people who buy something will only have 20's or 50's to buy a 50 cent item with.
Guard your money carefully. If possible, wear a tool apron or something. Never, ever leave a box of money sitting around. It will disappear.
Keep your house locked at all times. Don't let anyone use your bathroom.
Watch out for large groups of women of a certain island nationality. I don't want to sound prejudiced, but they are well-known in our area for descending on garage sales and pocketing anything they can. One of them will keep you busy by bargaining (offering you $1 for a $10 item and pretending not to understand you when you say "no") while the others are stuffing everything they can into their large purses. It's happened at every garage sale I've helped out with. I don't know if you have a large population of them, but we sure do here in Florida.
Don't say "But I paid $50 for it, I can't sell it for $5!" Rather say, "You know, those sell for $50 at Home Depot right now. This is a fantastic bargain at $15."
Don't feel insulted when someone doesn't appreciate your "treasures". It can get you down a bit at first when someone isn't seeing what you saw in the item.
Be prepared for dealers to show up a half hour before the sale is scheduled to begin, bang on your door and ask to see what you have (or even the night before....) They will then insult your items and low-ball the heck out of you. They tend to be rather unpleasant and it's very easy to figure out who they are.
As Elsa says in "Frozen": "Let it go, let it goooooooo........" Toward the end of the day, you really don't want to haul all that stuff back into your house or have to haul it off to Goodwill. Let it go.
Have a box of small trinkets, etc. to hand out to children for free. Tell them they can rummage through the box and pick out one item. There's nothing better for sales than a happy child. The mom now feels obligated to you....maybe....
Forget the cookies. Maybe have a couple of jugs of cold water and some paper cups. That will also make people linger. I don't know if you have grandchildren, but they enjoy selling sodas out of a cooler and keeping the profits for themselves.
Above all else, just remember: YOU'RE NOT GOING TO SELL YOUR ITEMS FOR WHAT THEY ARE WORTH. At least not what they are worth to *you*. That's not what garage sales are for. If you want to sell things for high prices, go the EBay route.
When pricing, always leave room for negotiation. Very few people will pay what's on the sticker. Don't price them too high, though, as that will cause a lot of people to just pass it by. Early in the sale, when someone offers you a price you don't want to accept, say "It's a little too early in the sale for me to go that low. If you want to check back later in the afternoon and IT'S STILL HERE, I might go down on that." If they really want it, they'll up the ante immediately.
Don't "hold" things for people unless they leave you a deposit. Or give them a time limit, i.e. if they "have to run to the ATM for money", they'll get a half hour and then the item goes back up for sale. Otherwise, 75% of them won't be back.
If you have a large item for sale (furniture, etc), be reasonable about when they can pick it up. IF they pay you in full, let them have until the next day or so to come get it. Sometimes it takes a day or two to borrow a truck to come get it. If someone needed delivery of an item at my sale, I sent them over to the teenage boys across the street to negotiate for delivery in their pickup truck. I think they were charging $20 for within 10 miles and a negotiated price for further. Everybody won.
As Gary said above, signs are essential. And not just a sign at the entry to the neighborhood saying "GARAGE SALE". Back when I enjoyed going to garage sales, there was many a sale that I could never find in a convoluted mass of streets. The sign on the main street should have at least the name of the street you live on. Every time there is a turn, put another brightly colored sign with an arrow "GARAGE SALE --->"
And, please take down the signs after your sale. People tend to leave the signs up long after the sale is over and you'll have people driving around the neighborhood looking for the sale when there is NO sale. I had a friend who had frequent garage sales and she'd just leave the signs up and put a sign on her door that said "NO GARAGE SALE TODAY". If she wasn't a friend, I would have kicked her for it.
Make sure you don't have to have a "permit" from the city to have a sale. Also make sure garage sales are permitted in your neighborhood. In my neighborhood, which technically is a condo neighborhood, garage sales are only allowed on one specific Saturday a year. People will try to have one at other times, but if Ilsa, the Condo Nazi is wandering around the neighborhood, she'll take down your signs. In a nearby town, you have to go to City Hall and "register" your sale (for free) and get a sticker to put on any signs. They have a city employee who does nothing on Fridays, Saturdays and Sundays but go around and take down any sign that doesn't have an official sticker on it.
Well, if that hasn't scared you off, have a good time and let us know how it went.