Lots of great tips on this thread! If you're holding a multi family sale, make sure the tag reflects who it belongs to, either by initials or color, etc. When I price clothes, I also put the size on the tag.
Organize! Many shoppers have commented my garage resembles a store. I label the sizes on the table and they follow an order. All toys are together, kitchenware, etc.
Have a partner--not only do you not want to do all the work, you have to go to the bathroom sometime. And while you don't want to hover, you certainly want to keep an eye on people.
I set up a little tv tray and chair as my "checkout". I also have on hand a calculator, pen, extra masking tape and my notebook where I track sales. Money I keep in my Disney World fanny pack on my person, it keeps me motivated (and money safe)!
If you are cash only, I would post it on a sign. I also post no bills over $20, and sorry, no returns.
Some sales have balloons or flags in their yards to attract and lead people to their sale. Like everyone says, good signs are a must, too.
Things that really seem to go are puzzles (make sure they have all the pieces), videos, any type of kids' educational toy, and housewares.
Good luck with garage sale season--they're a lot of work, but it can mean a lot more room in your house and a few more Disney Dollars in your pocket.