I go to a lot of garage sales with my mom (usually every Saturday in the summer).
You should have it organized. Put stuff that is related together. If you have stuff that you want to sell for the same price, then having an area of each price helps (like a $1 table, a $2 table etc). If you have books, try to have them set out so people don't have to dig to look at them, so one layer. You can have them so the spines are up, but don't stack them. If you have any electronics, be prepared with an extension cord so people can make sure they work. Have prices on most stuff, but be prepared to haggle.
Have change, most people will pay with bills so you need change to give them. You can go to a bank to get money. Keep the money on you both to make handling money easier and keep it from getting stolen.
Have some plastic bags available, some people might buy a lot of small stuff and don't have a bag of their own.
Have good signs, this is what gets people to your garage sale. Big bold arrows, the words 'Garage Sale', your address and the times. Where you place the sign is also important. Put the sign on the side of the street that people have to turn and try to give some warning, so place it back from an intersection so people can change lanes if needed. Try to put a sign at a major intersection near you, then at any turns. If there is more than one way to your house, then put signs from as many directions as you can. It is a good idea to have balloons or something at your house as some can be hidden by cars and are hard to see right away.
Weekends, especially Saturday, are the days that people tend to go to garage sales. Many people do work during the week or have kids at camp, so aren't looking for garage sales. Having it on Friday afternoon/early evening and Saturday morning/afternoon should get you more people. I know I only go to Friday ones if it is close by and on my way somewhere.
But I guess I don't have any real advice, I've never been in charge of the garage sales, I just helped out some. But good luck!
