I usually have 1, if not 2, garage sales every year. It is alot of work, but, the house gets cleaned out and you get to make some money. After the sale, I donate everything that didn't sell. Try to either call a charity to come pick up the left over items right away or drive the items over to the charity the next day so there is no chance that you will bring the items back into your house. Once you have chosen the items to sell, you have lost all attachment to them and don't need to keep them.
Price all items. I usually price the items as I find them all through the year. Then, I put the item into my garage sale corner and it is ready to go. So, when I set up for the sale, all items are priced and set up goes very quickly.
Make sure the items are clean and that electronics work. I do sell broken toys (not too broken) for a quarter because most kids will play with broken toys. I know my kids do. But, for more expensive items, make sure the item works or throw it out. If you do sell a broken item, make sure that you tell the person it is broken.
Always put up a sign that states: ALL SALES FINAL! NOT RESPONSIBLE FOR ACCIDENTS!
Try to price items as fair as possible. Remember that this is a garage sale and not a department store. Just because you paid $50.00 for something doesn't mean that it is worth it now. It is now used. You may get only $5.00 for it. Garage sale prices are meant to be reasonable.
Make sure to merchandise the items so that people can see them. Put bigger items out towards the street so that people driving by will want to stop. Don't put 50 books in one box so that you can't see any of them unless you dig to the bottom. It is better to lay them out on a plastic bag so that you can see them at a glance. I don't mind digging in a box. Sometimes the best items can be found on the bottom. But, if I have to spend a great deal of time digging through a box, I won't.
Remember to get a permit if your town requires one. Make a sign(s) for in front of your house and if your town allows it, put signs up around town. Put an ad in the paper or on Craigs list or anywhere else you can to bring people in. Have lots of change handy. I usually have $50.00. $30.00 in $1 bills, $15.00 is in 3 $5.00 bills and $5.00 is in quarters. This seems to work out fine for me.
If you state that you will run the sale from 8-3, make sure the sale is set up at least 1/2 hour before, by at least 7:30. I usually am set up 1 hour before I open officially and have made alot of sales by then. If you wait until exactly 8 to open, you will have lost sales. Noone wants to wait around for you to open since there are other sales on that day that they want to get to.
Make the day fun. If you have kids, let them sell lemonade and cookies and help with the set up. They will have fun being sales people and will make some money too. Go in with a friend(s) and this way there will be more to sell and maybe more people will stop by. If your neighborhood is having a sale, join in. People like to go to neighborhood sales where there are alot of sales in the same area. You will probably sell more at that time then if you did a sale on your own.
One important thing. Make sure to lock the doors to your house, especially if you are doing the sale by yourself. Also, keep your cell phone handy. I have needed to make calls for people whose cars have broken down in my driveway during the sale. You never know if you'll need your phone.
Good luck! Hopefully it will be a sunny beautiful day for your sale.