I have only had two garage sales, so I'm not really an expert; but, I made almost a $1000 in my 2 day sale, and $700 in my one day sale. So, I did get fairly good results! Both sales helped us get to WDW with extra money.
I think it is really important to clean all items thoroughly! I have been to a lot of garage sales where items are dirty, and I think that is a turn-off. If it's clean, it looks nicer!
For small items, ziploc bags are great to put all of the pieces together so they don't get separated.
If you are selling electrical things (TV's, lamps, appliances that plug-in, etc), make sure that you have an extension cord for people to plug them in to see how they work,
I do not think that clothes, other than baby/toddler items, sell particularily well. If you do have clothes, it works better to have them hanging on a pole. If you fold them and put them on tables, you end up with a mess and have to constantly straighten up!
Put like items together: kitchen items in one area, knick knacks in another, etc.
I like to constantly rearrange so things look good, as other items sell.
My parents, who sold the contents of their whole house a year ago, had free coffee and donuts holes. I thought that was a good idea!
Have plenty of wrapping, such as newspaper, bubble wrap, grocery bags on hand.
Garage sales can be a great time to involve your children. Selling homeade cookies, cocoa, lemonade, etc. You/the kids can make some extra money, and these things seem to go over well! Nothing beats the smell of homeade cookies! You can also make items ahead of time and pre-wrap them for sale (rice krispie bars, etc).
Also, I took some nice furniture pieces from my house to showcase some of my items I was selling. I took a corner cupboard, for example, and arranged a lot of decorative items on it. Those items flew off the shelves, and then I just stuck more stuff on them. You can put a "sold" or "not for sale" sign on any furniture you use for this purpose.
Think of other creative ways to showcase items. For example, quilts could be arranged on a clothesline.
So, those are some of my tips. I hope you have great results with your upcoming sale!