S.T.
My sisters, some of her neighbors and I just joined together and had one. Rather than repeat the stuff said below here's my tips to add:
1) There's a difference between the stuff that you just want to get rid of at any price versus stuff that's worth money. Price accordingly. Better to sell a piece of junk for $1 or less and let someone else take it than to have to pack it up and drag it to goodwill yourself. But if it's an old designer handbag hold out on price - post on craigslist or ebay if it doesn't sell at the yard sale.
2) Watch for theives! My sister's neighbor was selling Mary Kay cosmetics at the yard sale and someone stole a $13 item. We are pretty certain of the lady who took it since she asked how much it was, said it was too pricey and when other people arrived she hovered around the cosmetic area while no one really kept an eye on her! And she drove an expensive car too!
3) Decide ahead of time how much you'll settle for in terms of price. Many people really want huge bargains - they want to buy your Beatle's White album for fifty cents. You've priced it at $5. Decide if you'll go down to $2.50 or not. don't be afraid to tell people no if they want your mahogany chest of drawers for $5 if you really want $30. You can't buy a decent chest of drawers for less than $200 and even that will be made or particle board.
4) Run an extension cord so people can test the radio, iron or lamp to make sure it works.
5) Keep your money on you. You can get a pocket-apron for about $1 at Home Depot to keep your money in and on your body. AGain - theives abound - you'd like to think that they dont - but they do.
6) Use BROAD tipped dark markers for your signs - draw arrows - use simple directions such as "Right on Main to Maple Ave" - then hang another sign on that corner to take them "Maple to Beach St". They'll see the balloons or sign at Beach St and know where to go. If you see another garage sale hang signs directing people from that sale to yours.
7) If you can get a few of your neighbors to run a sale on the same day - even better! then the sign can say "YARD SALE - 4 families!"
8) Bundle things. We had a pile of Rescure Heroes to sell. I put them all in a box and priced it at $1 per action figure. People picked through bought one or two - at the end of the day I ended up with a box of rescue heroes, minus 3 or 4. In hindsight when someone bought one I should have thrown in another for 50 cents more, and a third for an additional quarter! Maybe I wouldn't have had 20 remaining rescue heroes. ;o)
9) Put the good looking big stuff closest to the street. I will drive slowly past a garage sale and if all I see is clothing, glassware, babyswings and toddler playhouses I keep driving... I'm looking for chairs, wooden furniture, etc.
Finally - don't expect to get rich on this. It's a lot of work. If you work outside the home, have kids and time is precious you may find a yard sale to be more work than it's worth unless you have a lot of desirable high priced items. When we were selling kitchen tables, chairs, cabinets and a double jogging stroller at our first sale we made over $500 and it was worth the time. But when we had a sale a few years later and made $120 over the entire day - it was barely worth the money for 2 days (the staging day before & the sale day) of my work.