I am going to post some tips for having a successful garage sale.
Advertise, advertise, advertise!
Get an ad in your paper
Use free classified ads like Craigslist.com
Post on community Forums or message boards
Post signs in your area but be careful : some jurisdictions have limits and rules. (check those first)
Be specific about the place, date, hours.
List items for sale.
When you are looking through things you want to sale try to box them together according to the type of item. If all clothes are in one box it will be so much easier when it is time to set up your sale.
Save your plastic grocery bags. You will need them to bag the stuff people buy.
Know when to have your sale- when do most people in your area get paid? That would be a good time to have your sale.
Look through things you will be selling- Make sure that clothing pockets and purses are empty.
Save all those metal hangers from the cleaners. They are wonderful for sales. I do not use the cleaners so I ask my friends to save them for me. You can also buy them by the bundle at some thrift stores.
Clean items up. Clean all dirt off items that you have for sale. Things will sale better if they are cleaned up.
Bundle like items. If you have 25 pencils do not try to sale each one. Bundle them together and sale at one price.
Get Change. You will need lots of ones and fives.
The Day of the Sale
Expect early birds! If you do not want early birds say NO EARLY BIRDS when you advertise.
Make sure that every item is priced in view. You can lose sales by not pricing items. Try to put the price sticker on top of the item, not the bottom.
If the day of the sale is going to be super hot consider selling sodas or bottled waters.
Play some background music. It allows people to talk about whether they want an item without having to whisper.
Put your big ticket items near the road. It will help draw in more customers.
Be friendly and talk to people.
Have electrical outlets nearby to plug in electronics. Customers will want to make sure they work.
If you get an offer on an item that you aren't ready to come down on, offer to take their name and number in case you don't sell it.
Good Luck with your sale! If you have other tips please post them in the comments.