You can get tired, cold or hot according to the weather, hungry, thirsty and impatient in the space of a few hours... then the hours, which seemed perfectly acceptable when you wrote the ad, can drag on and on and on...
So the first rule of order is to make yourself comfortable. Cook ahead of time or plan on having sandwiches. Plan on sharing time "on the floor" with someone else. Never have a garage sale alone. You'll need someone else to take over while you eat or answer the phone or change into warmer or cooler clothing, but you will also need to take breaks without having to do anything.
Wear comfortable clothing and comfortable shoes, have your favorite drink on hand and get a good book going for those times when no one is around but you can't leave. Turn off the cellphone and minimize interruptions when there are people around. Pay attention to your buyers, but don't follow them around or make comments about what they are looking at unless asked and don't make them feel as if they're being watched. Make it your goal to make them feel comfortable. Small talk is great, but don't talk so much that you detract from the business at hand. Give them room and time to browse.
If you want a successful garage sale, timing seems to be more important than anything. Watch your area to see what day of the week most garage sales are held and plan on having it near the first of the month when a lot of people have more money to spend. Fridays seem to be good for that reason, too.
Ten cents on the dollar is the going rate in most places, but adjust that according to the amount of wear, popularity and style of the item. Consider whether it can be a collector's item. Watch for store hawks - people who buy at garage sales for the purpose of reselling in a second hand or antique shop. If you want to sell to them, it's ok, but be cautious in cutting deals for them as they usually really know how to get a good bargain - at your expense. Be extra cautious early in the day when you still have opportunity to sell at a better price.
Don't be afraid to haggle, but know your limits. If you're in it to just get rid of things, you might as well haul it all down to the Salvation Army, but if you get too greedy, you won't sell anything at all.
Advertise your most interesting items, put up plenty of signs, make sure your sale is easy to see once the shopper gets close. Relax and smile. Smile and relax. You'll sell a lot more with the right attitude than with all the planning you can do.