Usually Memorial Day and Labor Day are big yard sale days, but I have noticed it depends on your area. Let's be positive and say your area is big on yard sales these days. Labor Day is just around the corner. Saturday will be the day you want to set up and be prepared to make some sales. Sunday and Monday not so much. Start getting ready now.
Put a box in each room. As you run across stuff you think you might want to try and sell, put the item in the box.
Create a yard sale "kit" box: tape, markers pens, card board and poster board (for signs), change (coins and bills), bags (grocery type plastic) and any other knick knack you think might help the sale go smooth.
Pick a place and arrange ahead of time to use it. This is for people who don't have yards or have a yard too far off the beaten path for a successful sale. Ask a friend if you can use their yard or ask permission from a business that may not be open on Saturdays if you can use their parking lot.
In some areas there are abandoned lots on the side of main roads that have been adopted by people to set up for yard sales on a regular basis. If you plan on going to one of these areas, show up early to get a spot.
Price things while you have time on your hands between now and the sale day. Individually price or plan on a group of things being a dollar (or whatever) each and all placed in a box dedicated to that price.
Find something to sit on and have it handy: lawn chair, sports chair, or stool.
If you are going to do a yard sale with some other people (same family or different), make sure they are aware of the time table. Ask them on different days how they are doing with collecting and pricing stuff. This is a friendly way to remind them and place a passive kick in the pants for some.
If you have big things you want to sell, you can hang a sign up instead of moving the item to be seen. Like if you have a grandfather clock, place a sign out that says "Grandfather Clock for Sale" (or the price). If someone is interested, they will talk to you about it. You can show them the item, or arrange to show it to them at a later time if you are set up away from home.
Be wary of customers who show up in groups that spread out all over the place. Especially if one tries to distract you with a lot of questions about something, but doesn't really seem interested. They may be keeping your attention for others to take things.
And speaking of taking things, look inside cases and boxes of items people want to buy to make sure they aren't trying to arrange for a freebie or three. If you open a CD case and it has 2 CD's in it, be polite and say, "Oh, that belongs in another case". Or if you open a box and it has several things in it say, "Woops, I forgot to get the rest of this stuff out of the box". You don't offend anyone and you may still get the sale of the original item.
If you are going to another location for your yard sale instead of your home don't forget to pack a lunch, maybe some snacks, and definitely some drinks. This removes the desire to pack up early or leave and get something and come back (even if someone is there to watch your stuff). Your objective is to make money, not spend it.
A yard sale is a lot of work but it can be fun if you plan, prepare, and execute in a timely manner. Good luck :)