First, give yourself plenty of time to go through the stuff you might want to get rid of. It does no good to start cleaning out the closet and remembering that dress you wore to prom and all those memories it holds. This is stalling, folks. Instead, when you are looking through an area for possible sales items make 3 piles: It Sells, It goes back in the closet, Maybe -I'm not sure. Put the Sell pile in a box, and after the Maybe pile clutters up your room for another week or two, chances are - it will go into the Sell box also.
Hint: I usually pretend I will be moving 400 miles away. I hold each item up and ask myself - is this important enough to pack, store, and take with me? If so - keep it. If not - in the sale box it goes. After awhile, you get better and better at this. You start looking at everything you own with a critical eye: important? or clutter?
Next: When your boxes marked 'garage sale' start taking up quite a space in your garage, it's time to start thinking of a date.
In our neck of the woods - Fridays and Saturdays are the best garage sale days, but check around and see what works for you. Figure out how you will advertise your sale. In a small town, newspaper classified ads will bring in most of your buyers, but I have seen signboards at post offices, churches, and even Craigs List as places to post your sale. Make sure you state your days and hours plainly, as well as what you have (dishes, antiques, books, tools, etc.) If you live in a high-traffic area, you might get by with just signs (more on that below.)
Next: and I cannot emphasize this enough, start marking prices on your stuff. Give yourself lots of time to do this (not just the night before the sale!) You will miss lots of sales if people have to ask what the price of an item is. Buy some price tags (the sticky kind work well, except for books) and keep a stash of pens handy.
Do not mark things high! People come to garage sales to find deals. Yes, expect some hagglers, this is common practice. But most things go for pennies on the dollar. If you are unsure of prices, stop in at a few sales in the weeks before yours to get an idea.
Next: Make your signs. Here's where a little extra work will really pay off - big time. If you just put one sign at the bottom of the driveway, you will get the people coming from your advertisement, and maybe a few drive-bys and neighbors. If you take a little extra effort, buy bright colored poster board, or get some stencils and spray paint and make a bunch of signs (even that just say SALE with an arrow) and put them up (early in the morning of the sale, or the night before if it's not raining) at several major intersections directing traffic your way, you will probably increase your sale attendance by double at least. Yes, we have done this!! Yes, it works!! Drive traffic to your sale!!
Okay - you have all your stuff laid out on tables, etc, and priced, right? You have your advertisement placed, your signs up, you have plenty of change to give back to people when they start shopping. You have your coffee pot ready, a box of chocolate donuts to get you through the stress of the day, someone to help you (of course) plenty of change for customers, a stack of boxes or bags for people to load their stuff in, a comfortable chair to sit in. You're all set, right?
Be prepared for an onslaught! People will come early. (And this is where you will be in big trouble if you are not Prepared!!) If you say you start at 8:00 AM, be dressed and ready for a line of people earlier than that, cause that's when the dealers get there, and they will walk through quickly looking for stuff, buy what they want and leave. Then you get the regular garage sale people, and finally (have you taken a breath yet or had time to eat a donut?) you will get the late birds, the ones that stroll through and want to buy Something. Anything. It is totally amazing how much of your JUNK (I mean treasures *cough*) that people will give good money for.
So have fun. Make sure you follow all of these directions. Have a plate of free cookies for your customers and play a little music in the background (it puts them in the buying mood.) Put a smile on your face. Be amazed at how your garage empties out. Then relax and be glad it is all over. (We are still recuperating from our recent sale.)
Let's see, have I left anything out? I'm sure I've forgotten something. Who else has had a successful sale, and what can you add??